I agree so far with what everyone has said. If you specifically said it was an adult-only party, he is automatically discounting your wishes. I would politely tell him that it is an adult party, and that a babysitter would be a great option if they would like to attend. Otherwise, as it is not a PG 13 environment, you will not be allowing kids in.
Besides, if this guy really cares about his kids, wouldn't he keep them from such things?! If you were all dressing up as Care Bears or Tele Tubbies, I could understand, but it sounds like it's going to get delightfully evil.
Thanks for the idea about the bathroom crime scene, by the way! I might just steal! haha

Leigh Clements
The Mystery Maiden
Shot In The Dark Mysteries.com