For me personally I always accomodate the families I have in care before any potential new families. So in your case I would ask mom for a set date in writing. Tell her you are starting to advertise, need to know an exact date as you have families interested. Once you know their end date I would start advertising for that date, tell any potential families that is the start date. I would never term just because something better came along sooner. That is just me. No family is a guarantee and no child is a guarantee to be a wonderful child. New family could potentially pull out last minute, not work out etc and you could be left trying to fill spot anyways.