I don't.
I went through a similar period with a few families leaving and no one even commented on it. I don't even know if they noticed. I have only ever had one family ever mention something about a family that had left and I just told them that the particular family was no longer with me anymore. Left it at that.
I don't think that you owe them an explanation. I think if they know the other family and child, and have seen them in action, then they can probably guess why they are no longer with you.