Board Thread:General Discussion v2.0/@comment-27394201-20151213195721/@comment-26521006-20151215035223

Velocity Chicken wrote: It's not explicitly stated that they're children. However, the box of dusty children's shoes in Toriel's Home, seem to imply the other humans were children. Toriel took care of each of them, but they all eventually left, and met their demise. So, it's somewhat implied that the others were children. Toby Fox kinda confirms that humans souls were children in an interview that he did sometime when the Undertale Kickstarter campaign began.