I wouldn't terminate. I would tell her that while she is welcome to bring her child part time she will need to pay the full time rate as that is what you agreed upon when she signed on. If she goes, she goes. If you don't want to lose her, then be flexible. Or you could tell her that you will let her go part time but you will be actively looking for a new full time client and when you find one, she will lose her spot. OR, if you don't want to lose her, let her go down to 3 days a week.