Chara isn't happy about this, not at all. Still, they have something. The younger ones, the ones most vulnerable, not being approached, and while they doubt he might keep to his word, at least they have it so he can be beholden to it.

Sans, as an adult, can take care of himself, bones and blasters to offset one HP. Frisk for their part, nods and gives Gaster a thumbs up and... creatively paraphrases.

[They're okay with that. I know you wouldn't, too.]

In his right mind... well, this Castle has a way of twisting people's minds apparently. But Frisk is shoving that firmly in the Feelings Box.

[I. Um. Can go if you want now.]

But only if he wants. It's a problem, that they aren't in their own form which would make this so much easier, and they viscerally miss it with a strength that surprises even them.