Not a co-op game but you could try Shattered Pixel Dungeon. A rogue-like in the classic vein but with graphics and rules that make it approachable. My kids could handle it at 10 years old, but they did consider it "hard" then, and even still do a few years later. Open source, free on F-Droid (but donate if you like it), $5 on Apple app store. Actively developed for years. [Edit: Should mention that this is my favorite casual game on my phone, for many years.]
It does support seeds, so you could make it more of a mutual experience by both playing the same seed and talking as you're playing. That's not co-op, but not completely solo, depending on the kid.