Yes, as long as the child doesn’t have a disability that affects their ability to be potty trained. In other words, if the child’s disability is the reason why she is not potty trained you cannot immediately refuse to provide care.
Instead, you would need to see how you could provide a reasonable accommodation to care for the child. It may or may not be possible to provide care in your situation, depending on the needs of the child and the number of other children in your program.
Also, if you do accept the child who is not potty trained, you cannot charge the parent more for doing so.