In this modern day of firearms and other ranged weapons, it's best that they learn to close the gap as quickly as possible. A larger weapon like a No-Dachi is probably best for that, teach them to finish every fight with the first charge and swing is best for their health and safety.

Alternatively, maybe branch out and give them a Naginata instead, this is especially useful for a child as it relies more heavily on skill rather than pure strength, and the reach it gives is a huge benefit, especially since a child will have fairly short reach to begin with, compensating for their natural weaknesses.