Staples offers the Trailmaker 17" Kids' Backpack in seven colors (Black pictured) for $12.99. Plus, Staples Rewards members bag free shipping. (Not a member? It's free to sign up.) That's $7 off list and the lowest total price we could find. It features a double-zippered main pocket.
Also available is the Trailmaker 19" Kids' Backpack in five colors for $18.99 with free shipping (as above). That's $6 off list and the lowest total price we could find. It features padded adjustable straps.