My contract states "if I am open, you have the right to keep your child home, but you still have to pay for the day" just like yours.

I would tell them that it was her decision to keep her child home and still has to pay. If she doesn't pay by XXX date with late fees, then you will be have to reconsider weather or not you will be able to continue care.

Start looking for new clients, just in case she doesn't pay or decides to leave.

$35 this time + $35 every other time she decides not to pay. It will add up.