I remember Jane and the Dragon where Jane becomes a knight in Training. One of the aspects is that she is weaker then a typical Knight in training, just look at her, so her Knight that she is apprenticing for happens to be one that compliments her as she gets around her lack of power with skill and speed. Yet she doesn't skimp on the physical aspects of her training, she just needs to rely on a different edge.
While her rival is given the worse of the two knights (and honestly that knight makes me question why he is a knight...) because his natural aptitude at being a knight means he doesn't need much help. While early on he is somewhat of a villain, it is shown that it is mostly because he is being pressured to excel and tends to be lectured whenever he loses to Jane... yet is shown to be truly honorable.
I love Jane and the Dragon mind you. It has this way of making every character alive and interesting. Even the most annoying brats of a character (The King's son) has sort of bits of character that makes him intriguing, for example though he is spoiled, he is too soft or kind hearted to enjoy hunting... a hint that he may actually have what it takes to be a good kind king.
But still tuypo1 the issue is mostly "Brawn is good" usually goes without saying, even for a kid. The reason why most things don't go "Brawn is ok" is because that by itself means nothing.
It is about how to use force, when to use it, and for what reason.
Children don't "Grow muscle" (at least most don't) so they don't really deal with that.