Around the age of 10, he says. Depends on how dumb the 10 year olds are.
Best RPG for kids
With essentially no prior experience role-gaming, I started running Basic D&D for the kids in my neighborhood when I was maybe 11 or 12. Granted, most of that was me just pulling shit out of my ass, but it still shows that 10 year-olds probably don't need to be babied too much. With that said, RPGs are pretty daunting, and I don't think there are nearly enough good entry-level options for people in general. 5e is hailed for its simplicity, but it's really extraordinarily complicated for a first step into role-gaming (and only simple compared to its immediate predecessors).
Anyway, my recommendation if you don't want to go for something ultra-light (and kind of silly) like Risus, is Barbarians of Lemuria (see pic), especially if you restrict access to magic/mystic careers, which tend to be significantly more complicated than their more mundane counterparts. It's a swords and sorcery system anyway, and wizards being bad guys fits right into that genre.
mediafire.com
this
dungeon world
Mouse Guard teaches good roleplaying. It's very important for kids to learn that before minmaxing at dnd.
It would be perhaps better not to learn to minmax at all as the only way of RP.
Macho Women with Guns
Nah just kidding. Toon.
FATAL.
Ryuutama seems like it'd be pretty straightforward.
Overcomplex and generally bad.