What the behavior really is
Herding dogs were developed to gather and move livestock. The Border Collie, for example, came from the hills of the Scottish and English border, where shepherds wanted a dog that could read sheep and move them with eye contact and quiet runs. Other herding dogs, including some cattle dogs, work with their mouths too, and nip at heels to move stock.
Put a herding dog in a house with fast-moving children, a jogger, a skateboard, or a car, and that same wiring can switch on. Your dog may stare, crouch, circle, bark, and then nip. It looks like aggression, but it is often a chase-and-control sequence. The dog is trying to gather things that move.
Individual dogs vary a lot. Some herding-breed dogs never nip, and some dogs of other breeds do. A herding mix can also surprise you. This page is about the pattern, not a label for any one dog.
That said, the nip can hurt and can scare a child. It also may look like a bite to a neighbor. So it is worth taking seriously early.