Toy Story 1 and 2 were light-hearted stories without a lot of darkness. Toy Story 3, while rated G, has some serious adult overtones--gambling, deception, torture of toys by other toys, and lots of darkness. My 4 year old was scared and just couldn't understand why the toys were acting that way. We ended up leaving the movie.
I've seen people bring their kids into R rated movies, but I'm not like that. If you're not either, then I wouldn't recommend taking your young children to this movie. It's just too dark.