Whoever came up with the idea of carrying potable water within your backpack is or was a genius, that's all we have to say on the matter. Other than that you can make sure that you're kept hydrated the whole day long with clean and safe water with one of these backpacks from such great brands as Osprey and Camelbak.
Camelbak Kids' Kicker 50oz Hydration Pack 62131
Camelbak HydroBak 50oz Hydration Pack 1122001000
Camelbak Kids' Mini M.U.L.E. 50oz Hydration Pack 1155601000