Fans of various series written for children, including The Boxcar Children, will enjoy the activity guides and coloring pages on Albert Whitman and Conpany's website. This is just one of the many publishers who augment their books with online activities.
HarperCollins has a page just for young readers. Right now, you can find several sweepstakes for fans of various series to join. Click on the Games tab to find online games and printables based on HarperCollins' books.
If you like Junie B. Jones, the Berenstain Bears, Thomas the Tank Engine, or books about Sesame Street or based on Nickelodeon, then the site to visit is kids@random. There are individual sites for many of the series that Random House publishes.