A lot depends on if you are prepared to lose the family. If so then it is fair to make them pay full time for one of the spaces and a part time rate for the second space. Alternately ask them to put the one child in the playschool on all the days, keep the other child part time and fill the one fulltime space.

As for getting a fee increase out of them I probably would have done it when they put the one in playschool and asked for a reduction and have told them sorry no can do I am already subsidizing you enough as it is. For sure start there and say ok enough is enough I need to have these spaces earning income and then look at options. Not sure on the amount but can you talk to them first to work something out instead of suddenly hitting them with an ultimatum. It is still your choice of price and contract but it is always nicer if we can give parents the chance to do right by us before we legislate it in.