Here is a quick life hack if your or your kid’s water bottle won’t stay in the backpack side pocket: a bottle chain. I used a split ring (like those used for keys), a bit of aluminum chain (from my stash, but you can find light chain at the hardware store or hobby store), and an S clip (a carabiner would work too). Connect the metal bits together, then attach the split ring to the top of the mesh pocket (the reinforced part). The clip then holds on to the water bottle loop. The clip is easy to undo to access the water, but provides enough security that the bottle doesn’t slid out if the backpack tips over. I used to sew a bit of velcro on the backpacks for this function, but this metal chain is easier and can switch to a new backpack!