Since the tender age of four, kids are required to schlep their own stuff. We have used a variety of travel gear, from rollers to backpacks. Currently we are using North Face Recon Squash backpack. This version isn't terribly large, which limits what my hoarding-tendency kids pack, however, the Recon Squash has some convenient features like outside pockets that make accessing water bottles, ear buds, and kleenex easy.
Each kid is color coded...sounds crazy and it is, but I hate packing and color coding makes everything easier. Adele is a consistent blue and Leo a reliable green, the other two kids, Flora and Hazel, get the leftover colors, although Hazel gets the pinks or reds and Flora's impending emo stage has recently settled on black.