Since all requests need to be done in writing, I would email the dad stating that this is the contract we have agreed on and these days need to be paid regardless of the child's attendance. I would touch briefly on the importance of this for the child's well being as well as for your business. I would state that if this contract is no longer working for the family, that you will need notice of their termination.

If dad decides to show mom the email, that is up to him but at least you have in writing the dialogue between you and dad and the consequence for not adhering to the contract.