Part-time kids - can you switch days

I have 2 part-time kids (siblings). I was wondering what others do when it comes to part-time kids and their contract.

I require that this family pay for 3 days a week. Mom was on mat leave and the girls would come Mon/Wed/Fri. Sometimes she would ask to switch days on a certain week and I went along with it. If she used more than 3 days I would charge her accordingly. If she used less she was still charged for 3 but the days didn't matter.

Now mom is back to work and her schedule varies. This week I have the girls Mon/Tues/Wed. Mom texted and told me both girls were going to be away Wednesday and she was wondering if they could come Thursday. Would you then charge for an extra day? or would you consider it part of the 3 days a week?

Do you have anything in your contract that specifies how this would work?

I do part time care and personally I have no problem if a parent needs to switch a day but only, of course, if space is available. Otherwise they're out of luck! My contract states that the child's submitted schedule must be adhered to and that it is at my discretion for a day to be changed. 2 kids would be trickier to accommodate though! I don't charge extra for this but I make it clear that it can't be an all the time thing.

If I had the space available, I would have no problem switching days at no additional cost.