It depends. You can go with something like Madagascar which has humor that is pretty much at the child level, or Cars that has a little something more behind the jokes.
You can pretty much bank on this for anything animated:
Pre-1960 = absolutely safe.
Post-1961-1980 = Mostly safe, though some things are a little "advanced"
Post 1980 = You'd better watch it with them, or see it before they do. Depending on what you deem clean.
Honestly, I'd recommend watching the movies with them the first time regardless. Make it special, pop some popcorn, get out sippy cups with juice/chocolate milk, be prepared for short attention spans, turn off the lights, curl up on the couch and make a memory.
Once, we actually made movie tickets that we had to give to Daddy in order to see the movie. It was so much fun and a great experience. :)