Heh, you beat me to it.

And yeah, his reaction to Barty saying Selwyn's kids were there was "That explains--" (and then to tell Barty not to duel anyone else).

He knows it was them. I don't know if he was sure before that, but he knows now.
This should prove interesting.

I am slightly worried he managed to recognize Hermione, but she (obviously) isn't one of his students, so there's much less reason for him to. Which is good.