Depending on the Uechi dojo, youth\children will probably start learning this at around the 6-4 Kyu rank. If your child is learning it, seek out their teacher to learn how it is properly and safely practiced. It is important that they practice correctly and go somewhat easy initially because it is very easy to condition the wrong parts of the arm or hit too hard. The goal is to build up resistance, not injure and bruise each other. I wouldn't recommend practicing this exercise with your child, but learn to look for signs that it is being done incorrectly - as a parent safety should be your first concern.
As your child progresses, he will be learning leg and core conditioning, and "Sanchin Testing". Done properly all of these exercises will buildup a tougher body, which is a great defensive aid if they should ever be attacked. Conditioning is an integral part of karate with benefits that go beyond avoiding injury.