He has remarried and wants to bring his wife of one year. She used to call on her own, I put a stop to that. Yes - You have to allow her to come as well. You sound really threatened by her. You should be glad that she likes your children and wants to talk to them.
Children of divorce have a harder life anyway.
The way I figure it, the more people who love your kids the better. Someday, your children will be going to his house for visits. I know from experience, that if your exclude her now, that she will not go out of her way to include you when the kids are visiting them.