The Creature from the Black Lagoon (1954), its sequels, and the giant radioactive ant movie Them! (1954) and couldn't think of any '50s movies I had on hand that they might enjoy (I had forgotten that I also own The Beast from 20,000 Fathoms (1953)), so I recommended a '30s movie and put on King Kong (1933).
My daughters really enjoyed the movie, especially because they weren't expecting to see dinosaurs in addition to a giant ape. Despite having watched all three Jurassic Park films repeatedly, the girls are just as entertained by Willis O'Brien's stop-motion monsters as they are by Stan Winston's animatronic and computer-generated creatures. Of course, the funniest thing to me is that their favorite scenes seemed to be those that focus on the romance between first mate Jack Driscoll (Bruce Cabot) and Ann Darrow (Fay Wray) as well as those that feature King Kong fighting prehistoric reptiles. I guess I'm lucky that my girls like the giant monster violence in addition to the sappy stuff.