I would definitely talk to her now. In regards to the late payments, I would tell her that from now on if your payment is not on hand by the 15th, there will be a late penalty for everyday they are late, and you will not care for their child until you get your payment and late fees.
On the sun screen issue, I would tell her it is not your responsibility to tell her husband what your son needs for daycare. He should have read your contract and handbook, just as she has. Tell her you do your most to protect the children in the sun, but it is their responsibility to ensure he has sunscreen on in the morning before coming and you will reapply later on.
If you do not clear the air now, it may just keep going down hill from here and you don't want that. Let her know that you are the boss of your business, and if she doesn't like how you run it, then she is welcome to go elsewhere.