A lot of these kids aren't that great they just matured earlier. IMO that's what its mostly about. Almost all of the really good players are very large for their age.
But many of them have a lot of talent as well, and it always impresses me the # of home runs they hit off of that high a speed (because of the short distance), and with the small bats they use. The coaches definitely know what they are doing, because some of these players aren't big and play great.
But overall, the thing that impresses me the most is how most of them keep their composure so well with the millions of people watching them. part of that is from coaching too.