Board Thread:General Discussion/@comment-12036244-20160813193200/@comment-27701762-20160813195502

Monster Kid: "Yo! You're a kid too, right? I can tell 'cause you're wearing a striped shirt."

Toriel: "You are an... ... 'interesting' child."

"What a good child you are."

"Now be a good child and go upstairs."

"You naive child... If you leave the RUINS..."

"Pathetic, is it not? I cannot save even a single child."

"Frisk...You really are a funny child."

[Not to mention that she constantly refers to Frisk as "my child."]

New Home: "The King and Queen treated the human child [i.e. Chara] as their own."

"They thought that he had killed the child."

Sans: "on days like these, kids like you..."

[Not also counting the multiple times that Sans ends his sentences with "kid," justifying that as a mere colloquialism).

Gerson: "What are they teaching you kids in school nowadays...? Wa ha ha!"

"Try it, kiddo!"

We can also add in the fact that an adult human is so powerful that the power of all monsters combined would equal their strength. No adult human would die to the monsters in the Underground.

Frisk and Chara are definitely children.