Maybe for a child you know and have trained, buying arrows close to the child's draw length would be a good idea. I teach youth archery classes, and we make sure that all the arrows offered are at least 3-4" longer than the maximum draw length we expect, as a safety measure. Children get exuberant and do unpredictable things, like overdrawing the bow and jamming the arrow into the belly of the riser (or worse, their hands). Even my own grandson did that once, and I thought he knew better! Fortunately, I was at his side when it happened, and was able to grab the bow and the string as the arrow was bowing out to the side, and he was standing there with a look on his face like, "What do I do now, grandpa?"