Board Thread:General Discussion v2.0/@comment-27394201-20151213195721/@comment-27036118-20151215020439

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.