By letting them know that the issue is financial success for the daycare and nothing personal also gives them the chance to offer to pay for the full space even though they only use it a bit which of course they have no intention of doing since they only need a few hours not even full days come September but you have left that door open to them. For sure providing care till the child is back in school and then booking new families accordingly but I'm sure you had already planned things that way.