Yeah, I think the above comment is also underselling the strength of pre-teen children, or greatly overestimating the weight of arms, armor, and the strength needed to do things with horses. Children have worked on farms around horses ever since horses were domesticated. A sword or breastplate only weighs a few pounds, a knight could easily employ a pre-teen child to polish his armor, oil his sword, wax his boots, make his breakfast, or any number of other chores a knight doesn't want to bother with personally.