My 6 year old daughter has played Pokemon, Pokemon Conquest, and a bot of zelda.
Divinity: OS in coop could work too I suppose.
But I would also advise you to play Tabletop first. It is easier to get into because of the socail aspect, and is a good gateway to CRPG.
Not with a tabletop RPG, but with a dungeon crawler, either coop or not:
Heroquest if you somehow have it.
Probably not worth purchasing it, though as the mechanisms are not great.
Or its more recent successors:
Descent and Imperial Assault (aka Star Wars Descent).
These are probably too complex, but the gameplay is great.
Or one of the following coop games
D&D Castle Ravenloft
DnD coop dungeon crawler. But it lacks cool persistent mechanisms
Mice & Mystics
Same as above but with mice vs Rats and bugs. Worked wonders with my daughter. The game is a bit fiddly, though.
Of course, if you cannot borrow any, it might not be worth the entry cost (but Imperial Assault is cool anyway; and you can play it with grown ups).