I would call them... You could always say that you wanted to make sure that her son was ok as you haven't heard from them. That way you can find out what's going on and you sound like you're checking up on her child.

I hate interviews too, but I always do them. Even if I'm not actively searching. My business is filled by word of mouth and I have a small community I pull from. I always think that even if I don't have room for this client and they don't fit in from a wait list they will always have someone ask them if they know any good daycares, if I've impressed them in the interview maybe they'll refer me.