My son is 6 so I showed him Aliens vs Predator since there's little to no blood. I'll forgive him but he really likes the Predator ha. Afterwards he said, " Dad, the aliens aren't super scary.." I can't wait until he gets a little older and show him the REAL Alien movies.
Also watched Men in Black with him, which he loved. Geez, I remember seeing that in theatre when I was a kid. The movie still holds up and it just goes to show that a movie doesn't need to rely heavily on CGI.