Unfortunately some people see a favor as a sign that they can walk all over you. You let her pay late once so she seems to think it can keep on going.

Text her now. Tell her payment is due for the following 2 weeks at drop off or you will not accept her child into care and will consider it a breech of contract and termination will be effective immediately.

Any idea why she needed care immediately? Did the last daycare get fed up of her doing stuff like this?

Stand your ground now and terminate if need be or you are likely in for a lot of headaches.