Christians, WHAT makes children stop believing in Santa Claus and the Easter Bunny, and is this change good?

Religion exists because people are terrified of dying, and because there are snake oil salesmen who are willing to take advantage of that. When you're raised from infancy to believe that losing faith, or failing to brainwash your children the way you were brainwashed, will result in eternal punishment, you make sure you brainwash them.