I used to allow older sibling to go down to three day per week but not any more. It did mean take hit on 2 lost days and not being able to find someone else because it need be available for child return after mat leave.
Now parent have two choice :-
1. Older child remain in FT and parnet get first refusal if another FT place open up. That does not mean if one is available many months in advance, I hold it for free. If they want it, they pay for the second place or my ad go up. If they opt not pay for second place, it risk that another place come up when they need.
2. They leave. And they hope for 2 place nearer time they need. But they have same seniority as anyone else applying for a vacancy.