Kids will carry their own lunches and locks in backpacks. Coaches will have some extra supplies such as bike pumps, extra sunblock, first aid and snacks and energy drinks when possible to carry.
Backpacks can sometimes make a rider's back sweaty on hot days so you might consider adding a rear wheel rack for the option of adding side bags, baskets or a bungee a milk crate on top to hold a backpack and gear.