As always, Luke Wroblewski nails it:
When kids interact with software they explore and engage with anything that looks interesting. Especially if it looks like content. Graphical user interface components don’t.
Consider the example of Dr. Seuss’s ABC book on the iPad. The intro screen uses colorful blobs to bring attention to large hit targets. But tap on one of these elements and up pops a standard modal menu asking you to select from one of three options. Modal menu dialogs and kids don’t mix.
More at lukew.com.