For those of you who are confused, running races like this, especially when there are multiple length races in the same area, could typically have the same starting lines and finish lines. The difference is the 5k route loops back earlier, and the 10k route takes some extra detour but eventually loops back later, making a path twice as long.
So I can only imagine that the event was organized like a clusterfrack and both 5k and 10k participants started at the same time... The kid probably just went the wrong way somehow and took the 10k route. And beat everyone else that meant to do the 10k.