This is what I have in my contract "Unpaid fees are subject to suspension or termination of care unless reasonable arrangements are made and accepted by both parties". You should include something like that before they sign so you can terminate right in the spot if you need to.
If you give parents time to pay what they owe you you are also given them time to look for alternative care and leave you without paying. I would told them to pay what they own first thing in the morning or there is no care. Believe me, this way money appears like magic. They have no choice. They cannot afford to stay at home everyday with their kid if they aren't allowed at daycare. They have to go to work. I would also include in the contract that payment has to be in advance. This is the way I get paid by every family and I think most ladies here do, too.