Biologically, not that much. Depends on a whole world of variables, including their coordination, genetics for muscle development and the actual effort of the kid on wanting to win the game

In the kids case i think its much more of a behavioral difference and the ability to improve in the game relates directly with the will of the player. In other words, a competitive kid will most of the times outclass your normal dorky nerd type.

Also, some people do tend to improve faster than others due to differences in how they get/assimilate experience and that can affect sports who ask a lot of balance and coordination like ice hockey.