Not as such that I have found in my experience. I did a ton of searches for you, and there might be some home brews out there, but I didn't find anything beyond some mentions here and there, and the MMORPG.
I have a couple friends who like to run games for their children, and I've done so for my neices.
I am unsure of the age ranges, so take this with a grain...
So far, I find one of the easiest things to do is use the new Fate Accelerated Edition.
It works well with younger players, because it has approaches instead of normal skills or stats:
I am going run as fast as I can and beat them to the door...
Okay, roll your dice, plus Quick.
I am going to trick him... roll dice plus clever.
When it comes to aspects, just remind them to describe their aspects as if they were playing make believe.
The other option is to take something built similar, and convert it. QAGS put out Laser Ponies, which I know a lot of people love, and it's pretty easy to use.
I like the World of Dungeons route for conversions: Your character has whatever stats you want. Roll 2D6; 6 means the GM does something, or something goes wrong. 7-9 means a partial success. 10+ means success. If you have a skill in something, you cannot fail.
I find that my neices like that kind of action.