He can really only guess at what's going through their head, here. Frisk certainly didn't react like this the first time they met him, but...he wasn't the first monster they met. They must have known what to expect, under the mountain, but knowing and seeing can be two very different things.

He's even only mostly sure that this is his Frisk - their soul is still basically the same, but it's like the one he's familiar with is compressed down into and hidden beneath this younger, small soul. It's incredibly distracting.

Still, he's not about to leave them here. It seems like he's making progress, even if only because the shock is wearing off or whatever bruises they're developing are more painful than he is scary. Moving as slowly as he can, Sans moves around to sit down in front of Frisk, though he keeps the cupcake and the toy between the two of them as a metaphorical wall.

He taps a fingerbone to his teeth, obviously thinking hard, before clicking his fingers. Ah!

"hey, kid. why do skeletons hate winter?"