It can be because the humidity is too low, the tissues are more delicate than in older folks, or the kid has undiagnosed respiratory allergies, picks his nose, or plays too roughly. Apparently, the veins are closer to the surface in some children so that even blowing their nose too hard will start a nosebleed. And then there is the traditional situation, where the child has stuck something up there.
In any case, if a child has a lot of nosebleeds, it is a good idea to have a doctor take a look just in case a vein needs to be cauterized, or a bean or some other small object needs to be removed.