This is where their camelbak's come into play. I'll backtrack a little. We travel some with my parents being in another state and we've stayed in state park lodges and camped out a couple times. Each time we go I take my "go bag" or if we have to stay in the basement through a tornado watch I take my go bag with me. This year I replaced my old backpack with a nice Camelbak as we started going on bike rides that were longer than one water bottle. Got one for my wife and the Camelbak Skeeter for my son. He liked having it on and wanted to carry stuff like I did. I'll pack food and socks and such in mine so we can stop and take a snack break.
This year we added fishing and hiking so I'm upgrading his Camelbak to the next one up and getting one for my daughter too.
So the plan is to put a clif bar or two and some trail mix or something else for snacking. I'll get those small bags of mixed nuts or peanuts and we found these slimjim like beef sticks that also come with a cheese stick in the package. Kids like those and the Clif Crunch and Mojo bars. So I wanted to teal them what to do if they would get separated from us for some reason, first never leave their pack for any reason and second find a tree and sit under it and blow their whistle so we can listen to it and find them. Then between the water in their camelbak and some food they should be set for a while. I'm planning on doing with their packs like mine, add one item at a time so the weight gain is not noticeable.
We've done a lot of the various what if's as Sue mentioned (well no what if the babysitter does whatever since we don't have a babysitter), I just didn't put those details in the Camelbak post because I was narrowing the scope down to just Camelbaks.

They actually have their own "go bags" now, matching Dora and Diego backpacks. I have them keep a flashlight or two, each has a compass, my son my old one from when I was in cub scouts, my daughters was from a chik fil a meal, each has a small plastic binoculars, etc. They take them when we travel.
I would liked to have gotten the larger Camelbak Scout to replace their go bags with better packs but I was afraid they would put too much in them. So for now the smaller one will have to do. So I figured I would start there and start buying decent gear, small Leatherman, real compass, real hiking socks, lights, etc a little at a time then when they outgrow these packs move to the larger ones. I took the brand new bladder from his new one to put it in the skeeter so I could sell it with an unused bladder and keep his used bladder from the skeeter for the mini mule.