Have you told your current parent that you are starting to interview for September spaces and if she wants to reserve them she needs to sign contracts and pay the deposit. If she does that then ok. IF she doesn't then go ahead and interview and take whoever signs first.

Sometimes I consider how old the oldest child will be in the sense of if the child will only be with me for one more year when mat leave is over and I am not in the school district for the child then the chances of the mom not moving both kids is slim. Getting into school based care is very difficult here with long waiting lists so chances are she will be looking for a new caregiver in her area and for many they will only take a school age child if they come with a younger sibling that gives more of an income.

This might actually be the time to speak with the parent about how practical it is for them to return to care - forget how old the child is.