I would personally change it to Monday at drop-off for the week. I'm just updating my manual for prospective parents, and changed it to this:
Payment is received every Monday morning at Drop-Off time for the week. Payment is by correct cash or cheque – I do NOT provide change. You will receive a receipt. If payment is late, I will require a $20 late fee per day. Each day that you do not pay, your child will NOT receive care.