I would send them a written message (text or email) that they have not been following the contract they signed. For that reason, you will require payment for the day's outstanding (that they had booked for) and in advance for the 3 days next week, or you will not accept the child into your care next week. Stay firm, do not allow the child inside without payment. If they choose to walk away, so be it, wash your hands of them and chalk it up to a learning experience.

Some parents will smarten up after they realise they can't get away with it. The ones that don't, never will, so it's best to just not deal with them. In the future, give written warnings the very first time (late payment or late pick up) and charge a fee the second time. You need to set out in the beginning what is acceptable.