I would tell this Mom it is for health reasons that he must be in a pull up for nap. Regress? No it is much better to have the child feel bad for soiling themselves and for putting you in this position of having to stand your ground for the cleanliness of your daycare. I do not wash the children's clothes, that is their parents job.
Of course, like the other posters have said, if dcm had given me the threat of two weeks notice, I would have terminated her right then and there. Speak to me with respect, not threats.