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.