I remember the movie from my high school days. We all wished we could have a babysitter like that. Now that I recall it more, that movie really was a precursor to "The Hand That Rocks the Cradle". Got the seducing sitter, two kids and the mischief/revenge factor. They even both take place by the beach. Note that Diane Franklin (the sitter) is the famous "MCI girl".
Review of Summer Girl
