I let my 6 year old play on this all the time and they love it. The amount of stamps in the library is insane!
This is my 4-year old's favorite. It's the only thing we've let her use the iPad for and I have it set up so that saves of her artwork end up in my iCloud.
Found Cosmic Osmo on there, one of my favourite games as a child. All black and white and very trippy, but so full of creativity, exploration and wonder. Also my gateway to the Myst series.