If you don't already have one, impose a late fee and make it substantial, like 20 dollars a day, with a maximum of 3 days, at which time you can terminate immediately for non payment of fees. Write a memo or email to everyone, and tell them effective immediately that these late fees will be implemented. Also in the e-mail, make a point of mentioning that as clients paying for a service, it is their responsibility to pay on time to avoid late fees.

Here is part of the memo i sent at contract renewal time:
Please note that effective January 1st, 2015, I will be charging additional fees for late payments, late pick ups of children and care outside of regularly contracted hours as it is stated in my handbook and contract. In the past, I have been very lenient with late fees and late payments, which, at times, causes alterations in my plans (i.e. groceries, banking etc). Please remember to hand in weekly fees on the last day of care for the week (during regular daycare hours) to avoid any late payment fees. Also, please remember to pick up your children by your contracted pick up time to avoid late pick up fees.

Here is my late payment policy:
After closing time, the late fee will be added. ($20.00 per day). If full payment is not made within 3 days (including weekends and holidays), your child will not be accepted into care until full payment, including all late fees, is made. In the same way that a utility payment, credit card payment, etc is charged interest when late payments are made, I also charge for late payment. If a period of 1 week passes without full payment received, the contract will be terminated, the position filled, and the collection process begun. You will be responsible for any and all costs related to collection of the child care fees, including, but not limited to lawyer/court costs, collection agency fees, etc.

*If payments are frequently or habitually late, daycare services will need to be paid in full, one week in advance of contracted days of care. In that circumstance, I will not care for the child unless payments are made in full before care begins. I reserve the right to determine when pre-payment for care is necessary.

In the same way that your employer pays you on your regular payday, I expect fees to be paid on my regular payday. It would not be acceptable to you if your employer were frequently late with your paycheck. I expect the same respect regarding payment on time.