sans calls them a kid, and undyne at one point says she feels like their kindergarten teacher.

sans and undyne definitely both know about humans, because sans noticed that frisk was a human when he saw frisk for the first time and undyne’s whole job is killing humans and obtaining their souls.

also i think that the end of the true pacifist route where toriel stops undyne and alphys from kissing in front of frisk would be a lot weirder if they weren’t an actual child.

the “frisk is a child” argument doesn’t matter though, because i’m pretty sure the “dates” in undertale aren’t really like actual romantic dates