Try a Lock'n'Lock container- I've really come to like them for field gear. Water resistant, not 100% water proof, but they'll take a dunking. Where they shine against tupperware/rubbermaid is that the lid is MUCH harder to come loose accidentally.

And as someone with a *cain allergy, +10 on making sure that you know who is allergic to what. Talk to your troop leaders, and have them talk to parents, see if you can get, say, a 3x5 with allergies and chronic conditions for everyone in the troop, then stick their cards in the bag, so you can KNOW.

And in here would be good place to stash that spare sunscreen and bug dope that should be with every troop outing, and maybe a bottle of PA or a flat of AquaMiras. If you are going to be the troop medic, push water treatment and field sanitation.