The classic child's backpack by Fjallraven (the Kanken) comes in lots of colors (they make a mini version too, for very tiny tots). Available from Fjallraven's website, Crewcuts, and various independent retailers.
And one more from Herschel - cheery yet smart!
This canvas one's by Baggu and comes in a variety of solid colors as well
I love this cute print; the pack is by Little Ella James
A basic yet cheery Lacoste pack
Ooh, shiny! This one is from Porter
I can barely stand this little adorable one.
This pack by Laura Spring is adult-sized, but would look so cute on the right teen...
And back to Fjallraven - this one would be great for an older child