I'm slowly working on a similar app. It's not polished enough to even call it a beta yet, but you can check it out at [1]. I've got a nice tool plugin API, but my graphics system needs work. Improvements are in the works.
It's been so long that recalling all the reasons KidPix made me happy would be impossible. But I can remember spending hours using that app, doodling, making little movies, and just enjoying everything about it. Little touches like the "Oh No!" sound effect made the app so much fun to use.
The app was bought (along with the rest of Broderbund's IP) and resurrected a couple years ago:
(I love KidPix)