It's all useful information, and Sans stashes it away until later. Things are a bit overwhelming at the moment, as it is. He doesn't really get what is meant by all the food rotting, but he'll probably ask Julien about that instead of the little kid.

Instead he'll answer her question, first.

"Yeah. I saw you get dumped in the hole, and the Prince showed up before I could do anything about it. Then I guess he... stung me? Some kind of tail thing. I didn't really think a skeleton could get stung, but these faeries will always find a way, huh?"

The levity he tries to phrase it with is entirely out of place. That entire encounter was extremely troubling.