I remember doing this with Shawn and Christian when they were little. We had a huge bird poster taped to our sliding back door and they would sit, watch and tally what they saw. It doesn't take a lot of effort and its fun.
The link in the title will take you right to the site. This link will take you to an explanation of the GBBC. What is the GBBC? another great and free resource is a site called All About Birds. Has information overload on all things birds!