For that age, a whistle and instructions on how to remain safe are probably the most useful. A jacket and water would be nice, but will they always keep these with them?

They should both have a info card on their bodies at all times. Name, address, phone contact, which KOA they're staying at, meds, allergies, etc.

I'd teach them how to use a cellphone and them give them yours (if you have one). Put your work and home numbers on speed dial buttons and thell them "press and hold 2 to get me", or whatever your phone supports in this regard. Might be the last time you ever see your working cellphone again!