What do I do about a child who has been potty trained but has accidents and cries when asked to change?

The child's mother and I work at the same school but at different grade levels. I have her child in my preschool clroom and that child had good days and has bad days. The child has been potty trained, but as of late has had a few accidents in the clroom. Sometimes that child is good about going to the restroom on time, other times there's a puddle on the floor where the child stands. When asked to change clothes, my aide and I try to do so discretely and try to make it positive, but then the child will wail, tantrum, and scream: "I don't want to, I want mommy!" over and over again. We've tried to make frequent bathroom visits with the entire cl, just to not single the child out, but I still hear "I don't want to!" with hands across chest. I feel like a huge failure. Actually, I'm pretty sure I'm one. I don't know what to do anymore. Help!