I probably would just charge her the full time rate and let her bring the older child as often as she wants with the idea the unused amounts are to hold the space for the second child.

If the space is open now then say sorry I need to fill it for my income but then expect to lose the older child too.

You could try and find a a school age child to fill the space till June.

You could advertise the space as a drop in space while you wait

Personally unless you have a space coming open just about when she will need it then I would not promise nor hold because too many things change in a year such as what if another of your children leaves care suddenly and you get a chance to take a family of two kids. Are you going to say sorry I'm saving that space?