I try to limit the time my kids can play games on the iPad (iPhone) but it's essentially unavoidable.
I also sometimes use gameplay time as a reward (and somethimes punishment).
Here are some games my kids (5yr, 9yr) like to play:
Super Farter - they love this game, you fly around by the power of farts.
Whale Trail - a flying whale in the sky.
Turtles, Huh? - a collection of short and simple games
Perhaps some links:
Any suggestion of other games your kids like to play?