After Plants vs. Zombies- this is probably the best iPad App game for kids (and adults) that you haven't heard of. A friend had recommended it to me, and I have to say I am very impressed. The graphics, music, storyline is really great.
That said, this is an interactive puzzle game- the puzzles are not always so clear cut- it seems as if it was designed for parents and kids to play together.
It's well worth the price!
The World of Goo- iPad App