I totally agree with calling her as soon as he wets himself and saying "You are going to have to come pick your son up as he has wet himself and has no clothes". She will definitely remember to bring them the next time. As Crayola kiddies has said, I would also advise her that you had to spend a lot of $$ on new items due to her child having an accident. If you don't want her to come and pick him up, maybe advise her that if she refuses to have him in pull ups, there is a fee per pee? Every time he pees and you have to do laundry (sheets, clothes, carpet, etc) that costs you money. $5 per pee? Sounds a bit crazy but she might think twice about him wearing pull ups at Daycare.