The sudden movement and noise is enough for them to inhale sharply but--alright, that's not quite upset.

"Really?" They hold him a little more tightly. "'M glad he did. Sans should too, though. He loves you a lot, just like me and him." And they're going to talk to him. Later. For now, Frisk's just going to stay here and nuzzle their face against his ribs, if he doesn't mind.