On the Wing: American Birds in Migration is a children's book suitable for up to Middle School or thereabouts. Remarkably, this ten year old volume is actually fairly accurate and comprehensible, covering most of the major aspects of bird migration, discussion ecological patterns, mechanisms, and methods used to study the phenomenon.
It is written and illustrated by Carol Lerner, who has prodcued well over a dozen anture related books of similar (high) quality including Butterflies in the Garden and Backyard Birds of Summer. Since these books are all at least ten years old you can find them at very low prices in used bookstores (in meatspace or on line). They're all good.
See also this post at 10,000 birds on migration.