I asked on twitter “Which is the best D&D-like RPG game for kids (8+)? I’m after a game which a twelve-year-old could read and learn, and an eye-year-old can play.
I got these suggestions:

Old School Hack (free online) played really well with kids of that age.

Check this out:newbiedm.com/rpgkids/
Alternately, try thisdndkids.blogspot.co.uk

With ‘kids’ at our club we found ‘Ghost Busters’ worked well with them running it on their own. Or the old ‘Starwars’
the old west end d6 star wars rpg? out of print now but available on eBay generally

Dragon Age RPG might do, at least the first box. It’s very original red box D&D

Also, the #Pathfinderbeginner’s box is excellent.
Castles and Crusades works for me and mine, as does the new Dr Who RPG (though it’s not very D&D-like). Looking forward to giving Legend a try once I’ve had a chance to print and bind the PDF.
The old WEG Ghostbusters is a very simple system – makes Toon look complicated – a simplified version of d6 Star Wars that was also used for Men in Black.