They signed a contract with set days. They need a change, you can't accommodate the additional days. There is no reason the other family should be terminated because this family needs more days.

They need to either stay as is, or they go elsewhere. Just tell them as per your contract you cannot accommodate additional days as set out by the government rules for number of children in care.

If they leave, they follow the written rules in the contract for ending care. Stick by the contract and it shouldn't be complicated. It isn't personal it is purely contract and government rules deciding here.