Next time send them away with something like this: http://www.findmespot.com/en/
They are GPS tracking devices that can be used to send update or emergency messages so that you know someone on an extended trip to a remote location is ok. Since it uses satellites, not cell towers, it's pretty hard to find a place where they don't work.
I'm sorry you'll be anxious in the meantime. I'm sure they'll be fine. It sounds like a great camping trip for them.