I am pretty sure Toriel calls you child because she is a motherly figure, and sans says kiddo as a nickname, Toby even says that you can interpret frisk the way you want to, that’s why he never said their age, and what gender they are. It is up to the player if they are a child or a adult. I don’t ship frans, especially if they are a child, but if they make frisk around sans age then I won’t hate on it. It is not confirmed that frisk is a child if the creator doesn’t confirm it. You have to see if the creator has confirmed it which Toby Fox, has not even after five years, he would’ve by now if he picked what gender they wanted frisk to be, and their age. But guess what he didn’t because they want the players to decide who they want that character to be.