I would say this.....
Unfortunately next Wednesday I will be over numbers by one child. Seeing how you are averaging taking one day off a wk with xxxx do u mind taking this wed off to help me out?

If she says no then unfortunately I would think she has the right to the spot as she is the regular child and not a school age child leaving. One of the school age kids will have to find alternate care for that day. Just say sorry with two new ones starting I won't be able to provide care for xxxx as I will be over numbers.

Also I would SOOO be talking to the "sick" child's parents. Saying their sick days are putting you out financially. And either she needs to start coming or you will charge her regardless if she comes or not.

I charge regardless if the child is in care or not for any reasons!!