How possible is it that the reason why Dess and Kris went into the shelter (if they actually did) is because she wanted to 'get back' at Kris for pranking Noelle so much?
I'd imagine she somehow found out the codes for the shelter, maybe by asking Asgore or someone else who mistakenly thought it'd be fine to tell her, and that's how she gets the idea.
So she gets Kris to come with her by posing it as 'exploring the spooky abandoned shelter on the edge of town' and planning to scare the hell out of them when they're actually in there, but when they actually get in there shit either goes bad very fast or at a point where they can't easily get out.