My nephew (started playing at 5, he is now 7 years old) is good at mario kart, and any other racing game.
He also like Donkey kong and plateformer (Dora the explorer !), so it can be anything which captivate him.
he loves anything which involves mario, he is even playing some Mario and luigi RPG in english (he doesn't understand anything and doesn't even read, he is just happy moving and jumping around). he manages to do the battles and move around.
He doesn't seem to understand the purpose of saving the game progress
but he doesn't care doing the same step again and again. I guess he doesn't care about reaching a goal, as long as he plays.
well, I didn't give you any suggestion, except mario ! just try and see what they cares about.
let them try and decide
provide few different genre, nobody likes the same thing.
Ps:
My nephew is addicted to screens (tv and video games), not good at this age... I don't know which consequences it will have later. currently, he has issue speaking correctly, but maybe it's unrelated, he just love speaking in his baby language.
If you can, don't let him get addicted this younger, supervise his play session, play with him, etc.