The problem you are running into has two faces, both of which are that you are not the target audience.

1. The first problem is that it is a very meta form of satire. It kinda depends on you having played enough rpgs to have internalized how they work. The idea that killing the various monsters and gaining Lv. is objectively bad will not hit you as hard if you are an outsider to the genre. There's also lots of other little details you will miss the meaning of, like how Toriel leads you through the spike puzzle because "its too dangerous".

2. Undertale has a lot of messages woven together in its themes, but the loudest and clearest ones are "Your actions matter, even if nobody notices" and "People are fundamentally good, and you can make friends with anyone if you try hard enough". I don't imagine those are messages you need to hear, since you are an adult, but when I was in school I did not feel like I mattered. School is a rough place, and I think your kids really appreciate being told they matter and that they are loved.

I don't know about you specifically, but I think the problem might be that it's too meta. It subverts your expectations really hard but, of course, it will fail if you don't have any.

I really like this guy's videos so I'll take the opportunity to link him. TLDW: it knows it's a game and that fucks with our brains' way of experiencing fiction.

idk, I've never played it, only seen some fan-art on youtube. I like it too. I guess that the appeal is that the characters are all (or most) pretty relatable, some are very cute, and that's enough reason to like them.