As everyone else has said, she is within her rights to use the spaces she is paying for. Personally, I would NOT tell her that you had offered someone else the space she is paying for. As a parent, I would be ticked if I knew that the caregiver was charging me and also someone else for the same space. If this ever arises for me, I would just give the parent the option of not paying for the space on that day because you have someone else who wants it. It seems fair.