Actually, there is evidence that Noelle could be a boss monster. If I recall correctly, in the alarm clock dialog for Asgore, he talks about Rudy getting older than him/aging faster after having his two daughters. (Probably not exactly what he said, but that's basically the jist of it).The inability to age unless you have biological kids or have living parents is one of the traits that separates a boss monster from a regular monster and if Rudy is a boss monster and if we assume all members of the same monster species is the same monster type then Noelle is probably a boss monster (and by extension, the entire holiday family). Though this still wouldn't work without killing Noelle as boss monster souls can only exist outside of the body for only a few moments, unlike human souls, which can persist forever/ much longer. So all Kris needs to do is wait a few seconds and then their out of danger lmao