few diiferent reasons, 1 kids move differently to adults, their movement is much sharper and clumsier, and they often make alot of noise too.i had this one explained to me not long ago too, which i never thought of myself. Children, particuarly toddlers communicate alot with their eyes, they will stare, and we all know what a stare is to a dog. A challenge and threat. Added to this stare, a toddler will also rush at a dog, we as humans know it cause the kid is pretty excited but coupled with the stare and a flapping of arms and a charge to the dog............to a dog thats not used to this all the dog thinks is "oh S#!T, this human is going to try and kill me or hurt me, what do I do? Fight? Flight?"