Here is child’s felted backpack with single row stripes in child-pleasing colors which race around the body.
Start with a square bottom and knit the body in the round. Close with a button and buttonhole in the flap. The straps can be adjustable with D rings or a fixed length, as you prefer.
Felting is not an exact science. Your final dimensions may vary slightly.
Yarn: Worsted weight wool in 5 colours, 120 yards (110 meters) of black for the bottom and straps, and 90 yards (83 meters) each of four bright colors for the body.