It's wonderful you've found a babysitter that your kids really like. If she won't take the "hint" about the movies, then you need to be more direct about what you want.
If you say it matter of factly, reassuringly, and without judgment about the past, then she won't be offended.
Here's a little role playing example:
mom: "_____, my kids just love you. Did you know they wake up asking for you after you've been here. It's not everyday a mom feels so good about her choice of sitters as I do you."
Then, when it's time to give instructions for the night -
mom: "you know, I really appreciate how you try to think ahead and bring something fun for the boys to do while you're here. I havent said anything before but I'm just not a big fan of Scooby Doo. I think the boys are still too young for the scary stuff. Would you mind if we tried something different than movies tonight?"
babysitter: "gosh, I'm so sorry, I had no idea!"
mom: "oh, no worries - you know how much we all love you. Here, I didnt want to leave you stranded for things to do tonight, so I put together a "Fun Bag". There's some puzzles, crafts and games in there and a snack recipe you all can make together. It's a beautiful evening, would you mind walking the boys to the park before dinner?"
I hope that helps you understand that a change like this doesnt require confrontation. If you are reassuring and model what you want, then that should be enough.
Good luck!