Congratulations!!! It might be worth your while to have a policy in your contract with the parents stating that you require 1 months notice on or before the 1st of the month with termination starting on the first of the following month. If they are late getting the letter to you then they pay for the extra month. One of the daycares I know of had the same situation happen to her and she took the parents to small claims court. It cost her very little and with $900. owing to her it was worth her while. Her contract stood up in court and the parents had to pay her $900. plus the cost of hiring a substitute caregiver for the day she was at court.
Even having this in the contract will make parents take notice and hopefully this does not happen again.