I'd be more suggestive of a walkie talkie set-up as you can buy these pretty cheaply at Radio Shack, or the like. 10 mile radius, which should cover you adequately (and if not, you're already in trouble). It is the approach my "in-law" family (of eight) use when skiing - they all just carry a walkie talkie.
Just a suggestion, as otherwise I can see you paying a fair amount for each sim card and plan.