What you do is then refuse any further care until she DOES pay you what is left to pay. So, let's say she paid you on Monday for this week - you KEEP that money and don't allow her to come thursday or Friday.

My contract says that the second you give me notice you owe me the remaining amount of weeks FEES that day in order to get any more care. That way, if they don't pay me I won't care for their child for the duration of the stay they still need.