If you are charging them a full fee for the week then she is technically entitled to bring her child all 5 days. The fact she "normally" only uses 4 days is a bonus to you. In this case she is in her right to bring the child on Thursday instead of Friday if she wants to.
See, this is something I couldn't figure out - whether I was being rational or not. Problem is, I only take four kids on any given day. I will not always be able to accommodate her on the Thursday. So, how do I word this?