TLDR
Ive added support for Favorites, hiding items and options to ES, would you like to test?

Background
My use-case for Retropie is not only to play old-school games like its 1999, but also to show these games to my two kids (age 5 and 3). I am really looking forward to using my Retropie setup as an informal museum showcasing the evolution of computer games in the period 1975 – 2000. My goal is therefore not to have all possible games installed on the system, but rather to create a well curated collection, including descriptions, box art for all games. I believe that ES + Retropie will allow me to build and maintain that, but currently there are some features missing.
For instance, it is currently not possible for my kids to play unsupervised with my small bartop build. When attempting this, it becomes painfully clear that ES and Retropie are both designed by geeks adults, for geeks adults. It took my youngest a mere 30 seconds before she was deep in the ES config settings. She does not understand what a scraper is (she does not even speak English). Suffice it to say that when she was finished “playing” things were slightly less well organized then before…
So I set out to make a feature request for ES, maybe bribing a dev with bottles of whiskey. After finding out that the EmulationStation project is currently on hiatus, Aloshi having an actual job etc., I decided to try and do it myself. What then happened was a black hole of github, man pages, C++, compiling on the pi and loads of other fun activities! Fast forward a couple of months of excrutiating pain hobbying…
The result of these efforts I would like to introduce to you now. There are a number of new features I have introduced: UI modes (plus unlocking), favorites (merged in from this PR), hiding game entries, hiding SW options.

UI modes
Central to this excise was to improve ES by creating some kind of user differentiation. My kids do not need to edit the metadata for a game, nor will my visiting friends. Having all these extra features around really complicates an otherwise very well designed UI, and I would rather hide these features when they are not needed.
For my use cases, I can think of three modi operandi that make sense:

1. Full user interface (aka admin mode) – This is the current UI, it has all the options, bells and whistles.
2. Kiosk mode – This mode hides most of the ui options, such as CONFIGURE INPUT, UI SETTINGS, SCRAPER, but also EDIT THIS GAMES METADATA, etc. In addition this mode also hides those entries in the gamelist which have their metadata-tag ‘hidden’ set to true.
3. Kid mode – This mode has all the restrictions of Kiosk mode, but also -only- lists games which are explicitly white-listed. This can be done by setting the metadata-tag Kid-game to true.

Ive implemented a new menu option under UI OPTIONS:

Getting back to the Full UI
Ok sounds good, maybe? But once you are in one of the restricted modes, the UI options (where I put the UI Mode selector), is not shown anymore. So how to return to the full UI once your in Kid/kiosk mode? Well one way, is to go to the /home/pi/.emulationstation/es_settings.cfg file and edit the UI_mode value manually:
<string name="UIMode" value="Full" />
I found this a rather cumbersome way, but it is good to keep in mind as a backup. As an alternative, I’ve implemented an input listener that tries to catch a predefined passkey sequence that you can enter while you are anywhere in the ES ui. The default passkey is [up,up, down, down, left, right, left, right, b, a] which some of you might recognize as the Konami cheat code from olden-times. This can be changed in es_settings.cfg if need be:
<string name="UIMode_passkey" value="uuddlrlrba" />
Now, if done properly, this mechanism should trigger all kinds of list-updates, but I am not sure which ones, or how to do it atm, so I just save the setting, and dump the user to the console/prompt, so you better have a keyboard nearby to get back in ES. (This is on my todo list).

Favorites in ES
So if you have been paying attention, you might have spotted three new metadata tags for the gamelist.xml files, plus a method to filter the displayed list based on those tags. I will admit I had no real clue how to go about implementing something from scratch. Luckily, this part was greatly simplified by kaptainkia, who submitted this awesome pull request for a favorites system in ES. Much of the work of what I needed to do was prepared for me. Great! He/She even included an option to display only favorites, while hiding the rest of the games! Wow!
Another feature that I needed to introduce was a way to hide entries from the game lists. This is the negative counterpart of showing “favorites only”. In this way approach, we would like to show all, -except for- those entries that are unwanted.
While the Full UI will still show all entries in the gamelist.xml, the Kiosk and Kid mode uses the ‘hidden’ tag to, well, hide those entries. For example, I plan to make all entries in the RetroPie ‘system’ hidden.
So this is the current situation, comparing the full UI with the kid-mode UI, using the same gamelist.xml:

Gamelist.xml and theming
So in three new tags have been introduced: favorites, kid game, and hidden (all boolean).

<game>
	<path>./BartVsWorld.zip</path>
	<name>Bart Vs World</name>
	<desc></desc>
	<image></image>
	<thumbnail></thumbnail>
	<rating>0</rating>
	<releasedate>not-a-date-time</releasedate>
	<developer>unknown</developer>
	<publisher>unknown</publisher>
	<genre>unknown</genre>
	<players>1</players>
	<favorite>false</favorite>
	<kidgame>true</kidgame>
	<hidden>false</hidden>
	<playcount>4</playcount>
	<lastplayed>20151210T220327</lastplayed>
</game>

Of course, these are currently not present in the gamelists you have, but once you edit the metadata for a single game, they will be added for that specific system’s gamelist.xml. If you want to edit your gamelist.xml files in some other tools, you will need to take these new tags into account.
Another thing to remember is that the visualization for these tags are NOT included in the current themes out there. They are easy enough to add, but it will need to be done if you want to see the status of these tags in the detailed-list view. In his/her original pull request, kaptainkia suggested to increase the xml version to 4 to indicate the availability of the new favorite tag. I would agree with this approach. At least from the ES side of things, you can then keep on using the old themes, without anything breaking. You will just not be able to see the values at a glance, or toggle them quickly using x for favorites and y for kidgames. When you open the metadata editor though, they are shown and editable:

Screenshot of quick-toggle options when supported by theme:

Note: When you try out this version of EmulationStation, it will rewrite your gamelist.xml files with the new tags included.

But did you know the old version does the same, but only with it’s own list of tags? Upon exiting EmulationStation rewrites the complete collection of gamelist.xml files, dont ask me why. That means that if you just spend a lot of time defining which files should be hidden or not, do -not- run the original version of ES, because all those changes will be lost ! (Don’t ask me how I know).

TODO:
There is of course stilll a lot to do, here is the list:
Must-Haves:

• Beta testing! There are bugs aplenty, and I would be most grateful if some of the more adventurous minded among you would be willing to take this for a ride and report back to me what your findings are. For more details see the section below.
• Runcommand. This is a bit of a sore spot. It is a really hacky implementation for something that really should have been part of ES from the get go. I understand it’s use, but it simply cannot remain in any kid-proofed system. One quick way to minimize the impact is to disable the joy2key part. Then you can only actually access the menu when there is an actual keyboard attached, which will not be the case when the kids are playing, at least not in the near future. Better would be to have two different runcommands, one for the full mode, and one for the kiosk/kid modes, that just takes the pre-set emulator+settings. An even better way would to allow the user to choose the core from within ES, but that will need some thinking.

Somedaymaybe:

• Add mechanism for showing / hiding image in a theme to quickly show favorite / hidden / kidgame boolean status. Currently only text output is supported (except for the rating component), but I think the detailed view is rather too detailed as it is. I am also considering introducing some simpler tags, such as:
  – year (rather than date)
  – a combined developer/publisher label eg: “Konami/Konami”
  – a combined times played / last played label eg: “played 23 times (last: dd-mm-yyyy)”
• Other sort options: Popularity (favorites first, sorted by last played date, then the rest, sorted by last played), favorites across systems. All these things realistically speaking need the newer sql database version of ES, which has never been fully deployed. I have had a look at it, but found it very frustrating to understand, maybe when someone else has time?
• Make sort type persistent: add settings option, small thing I found as a todo left in the code.
• Show some image of relevant controls when starting a game as a way to illustrate which keys are going to be used.

Beta testing / Giving this a try
So you want to give this branch of ES a try? Great! Here is a how-to list:

1. First, make a backup of every thing you are not willing to fix/re-install/re-scrape later.
2. No really, this is buggy stuff, do the backup.
3. Ok, you can find the repository here: https://github.com/zigurana/EmulationStation/tree/UI_modes_Kiosk_Kid_Full
4. To get this running on your pi you need to follow the excellent instruction here: http://emulationstation.org/gettingstarted.html#install_rpi_standalone up to the section “Compile and install EmulationStation”.
5. Then use the following commands at the prompt to finally install (this assumes a Pi2):
   1) git clone https://github.com/zigurana/EmulationStation/
   2) cd EmulationStation
   3) git checkout UI_modes_Kiosk_Kid_Full
   4) Pi2 specific config:
   ../configure --host=armv7l-raspberry-linux-gnueabihf --disable-pulseaudio --disable-esd --disable-video-mir --disable-video-wayland --disable-video-x11 --disable-video-opengl
   5) Now, build using:
   cmake -DCMAKE_CXX_COMPILER=g++-4.7 -DFREETYPE_INCLUDE_DIRS=/usr/include/freetype2/ .
make
6. You can now start the program from the directory where you installed it using (wait, did you really do that backup?)
   ./emulationstation

When debugging, start the program from the command line using:
./emulationstation --debug

Oh btw: this is my first contribution here, and my first encounter with Github, so if you have any comments/suggestions, please tell me!