I would call them and ask why they change hours on contract when that not the discussion. I see no issue in telling them that you choose client's based on contracted hours so there's no capacity to extend beyond the agreement. Tell them their child would be in first and out last and that you don't offer that length of day to anyone, especially an infant.
Until you sign that contract, so both have signed, it not valid anyway. This why I always send for them sign first. Otherwise, if I signed and sent for them sign, and they made change and signed, both name on it and harder to address.