How do I explain to my three-year-old what a Brit Milah ceremony is?
My recommendation is to tell them it’s a party for the new baby. I don’t think very young children need to have the surgical aspects of a bris explained. In general, I also recommend that children under the age of twelve not be permitted to watch the bris. I try to include the older siblings in the ceremony (before the actual bris takes place); you can tell them that part of the celebration is a big brother/big sister party.