Sports Illustrated Kids is a great all around sports site for kids. There are games, such as Ski Maniacs, that children can play. These are not necessarily all sports related, but it will still entertain them.
Then there are the more on point videos and photos of various sports stars. Competitive or creative kids can enter the contests in the contest section of the site. One such contest asks for the best funny caption for sports stars.
And of course there are sports themed articles, helpfully written in a kid friendly but still intelligent manner. All in all, a great site for kids who are generally interested in sports.