I held a spot recently for a mom coming off mat leave and I don't recommend it. In the future I won't even discuss it with a parent on leave for longer that 2 months. I now have a "waiting list" and I tell parents that if another spot opens up (reasonably close to their start date) I will contact them.
It's much easier to fill the spot now and be guaranteed the income, than to a) be losing money to hold her spot, or b) have the parent pay what you want to hold the spot and then feel obligated to continue working with a difficult family because you feel like you owe them.

I made the mistake in interviewing this family when I shouldn't have, because I really like them, that was the main reason for holding their spot, so for the future I won't even interview. Too hard to tell someone no at that point, for me anyway!