There are a couple of Mossberg 500 youth models that would be a good choice. I prefer the 500 to the 870, but I know not everyone does. If was to get another 870, it would be a used older one.
Another option is a full sized shotgun and then putting an adjustable stock on it. A hunting buddy bought an 870 a couple of years ago for his son, and put an tactical style adjustable stock on it so that he could adjust it to fit. His son loved that combo and killed quite a few deer with it.