fun is definitely the most important part. At those ages, if it isn't fun, it won't last very long.
My sons coach asked him to start doing 300 shots a week at home. He was not thrilled with the idea, but I go in the basement with him, and we make a whole game out of it. I put a goal light down there, and do his favorite goal calls when he gets a good one in. In a week he went from barely making it to the middle of the net, to clanking them in off the crossbar fairly regularly. I also got him to stop aiming at the dead center of the net
Now he does 200 a night, at his insistence. It's fun, and he sees progress, which fuels more progress. The important part for me is that he learns the lesson that practice equals results, and the important part for him is he laughs his butt off while doing it. Everybody wins