I charge a 2 week non refundable deposit at the time Of registration. Deposit is used for the child's last 2 weeks in care. When your new clients sign on with you, make sure you give them a receipt indicating that the deposit is non-refundable should the terms of your agreement be broken and non-refundable should the client decide not to start the hold in your care.
I also have my clients pay me every other Friday (they all pay on the same day) in advance of care for the following two weeks. If they don't Pay me I have the right to refuse care and keep the deposit.