On the other side of the coin since you only have two clients you could speak to the new client and ask them if they would mind if you included their name as a reference but also tell them that you understand they won't haven't been with you long and therefore what they would be able to speak to prospective parents is what it was about you and your home that drew them to you instead of somewhere else - just the idea that we felt this was the best place of everything we interviewed which may encourage the family to stop looking.