I highly recommend Cars for the little ones. I liked it too -- not an instant classic like Finding Nemo, but very enjoyable. I took my 6 and almost 3 year old and they both loved it. It was the little one's first time in a movie theater and he had a blast.
The 6 y.o. wants to go see Pirates but I told him no way since it's PG-13. I hate how they market to the little kids anyway! We got a Happy Meal at McDonalds the other day and it had Pirates stuff all over the box and had a little mini do-rag inside. I mean what 13 year old eats a Happy Meal? I wouldn't mind if it were an appropriate movie for them to see, but it isn't by a long shot. Same with the Star Wars Episode III promotional stuff. No way is he allowed to see that one.
OK rant over.