Our Stokes Beginner's Guide To Birds covers the 100 most common birds in the east or west and is organized by color of the bird so even children who cannot yet read, can look up a bird they see. The newly published, Young Birders Guide by Bill Thompson, III, is also a wonderful resource. Older kids who are already into birding should have one of the many full size birding field guides.
Kids should have appropriate sized binoculars, with smaller more compact binoculars given to smaller kids, and
full-sized binoculars for older kids.
The Young Birders part of the American Birding Association website is an excellent resource for kids age 10-18. They have a Young Birder of the Year Contest with prizes in categories for keeping a field notebook, bird illustration, bird writing and bird photography. They also have kid's birding camps and kid's scholarships to those camps and other birding activities. A Bird's-Eye View bimonthly newsletter is edited and written by young birders, (only $10 for 6 issues).
Most importantly, you can help spark an interest by taking a kid birding, whether it's your own kid, a grandchild, neice, nephew or just a friend.