Kevin Costner's character was way too calm

The guy finds out there's all this folklore that matches up to this specific scenario... the mound, the doll, his daughter's behavior, the mysterious injuries/deaths/disappearances, the old guy who killed his affected granddaughter, they find the body of the babysitter and he just has the teacher bring his kids back to the house.

When all signs point to your daughter being raped by some ancient ant-monster, you should probably be a bit more concerned about it. Even with the vomiting, the rash, the sleepwalking, and all of her other weird behavior, apparently bringing her to a doctor never crossed his mind.