I wanted her to have the best experience possible, so I devised a plan to take her ideas and convert them into the game without changing much along the way. I used a sample 2D game as a starting point so we wouldn't have to spend too much time writing code before she saw her creations in a working game. Note that if you don't deploy your app to the store, you can do all of this for free (plus the cost of Lego of course).
We started by drawing out the main elements of the game by hand. Here are the villain and hero characters as my daughter drew them.
After that, it was off to the Lego store so we could get enough 2x2 pieces to build whatever we wanted. Here's the villain Lego version my daughter created from her original drawing.
We designed the rest of the game elements including a tree and an egg, and then I used photos of the Lego concepts to convert her creations to simple sprite versions. Here's another character before reducing the pixels down to a simple sprite.
My daughter building Lego versions of the game sprites.
This is a screenshot from the first build I deployed to my iPhone (just for fun, I threw in a photo of my daughter as the player). Not bad for a 4-year-old with some Lego!