Miss Pink ;
This has not happened to me but you could do you contract like an employee when you earn vacation over time.
Put in your contract that vacation is accrued at x number of days each month. In the event of a parent leaving your care, if untaken vacation days are owed, then payment for those days is owed.
I get paid leave. I get 12 days. I don't do it like this. If they leave before I take my days then that's just how it goes. I don't take time off until Summer normal I have one week August and then odd days when I need. I do not think making more leave will fix this.