Gateway system for an 11 year old GM?

My kids play with a friend a fair amount and they basically do free-form pen-and-paper-style roleplaying while making Legos or other items. The terminology they tend to use is colored by MMOs, but I sense a budding GM of greatness.

What's a good system that's graspable by kids, but has the freedom/flexibility to have them "choose your own adventure" rather than getting super mechanical and rolling dice for everything? It probably also needs to not be a "kid" setting -- but obviously not so gritty it would iffy. (magic, swords, guns are all ok.. but exploring the dark soul of the human condition would be less ok).

The Cypher System is crazy simple at its heart. It is designed to be universal which should allow for play in many settings, and even universe hopping (a key feature of The Strange).

Hero Kids is a good one for kids. My daughter loves it

Check out "No Thank You Evil!". It's from Monte Cooke games and is a simplified version of Cypher System, made for kids, and increasing in complexity for kids as they get older. It also has child/parent interaction which is neat. It's not just for kids, it's for families.

Give'em a die with instructions that whenever an interesting choice comes up, roll the die. Four plus succeeds, three or less fails. Then, they can give point bonuses for good RP and good ideas and point penalties for especially hard things. Then they can transition to writing skills with bonuses on a sheet for characters.

Then they'll be ready to jump into Shadowrun.

I would actually talk to them about what interests them. My sons firsr RPG was Marvel Heroic because he is a huge Spider-man fan. Then we played Werewolf 20th because he was curious about werewolves. Then Little Wizards (which I highly recommend) because he was on a Harry Potter kick. It's the setting and subject matter the kids are interested in, I've found.

I like the idea about No Thank You Evil!

I'll pick up a copy and read through it and if it looks good I'll pass it along and report back!

I've played No Thank You Evil. My 9 year old daughter got it right away, and it is a surprisingly accurate version of the Cypher System, just redressed and simplified a bit.