I got a children's drawing that has to look like it's being paintet on the screen by a kid walking by. How do I get it to look the most realistic? Right now I've animated it with a mask with a little feather, but it doesn't look good enough.
You don't say which version of AE you're using. The new CS6 AE has a "feathered masks" option which allows you to put control points on your mask, and control exactly where the mask feathers are hard or soft. They can be animated as well, giving the ability to do much more "painterly" moves, albeit with a fair amount of work. By adding Dave's Roughen Edges plugin (not Dave's plugin, Dave's suggestion) you should be able to get a fine semblance of kid's paint work.