I don't have an OBi508vs so am not sure if it has a configuration to restrict calls to certain area codes.
I use a FortiVoice (formerly TalkSwitch) system for my work and it has the ability to restrict calls and limit calling privileges on a per extension basis. The cost of a professional PBX system like mine would be less than 4 OBi508s (phones not included) and if used with a SIP provider, like Steve suggests, would provide the lines and security that would likely save money and frustrations in the long run. Additionally, as a hybrid PBX, it can also be connected to a few OBi202s to use analog lines with GV. The benefit is that it can add a measure of security and call restriction on top of the OBis--assuming they were plugged into the FV PBX to add analog (in addition to SIP) lines.
How many concurrent lines does the OP really need? Seems like these kids are really being spoiled.
Personally, if I were running a camp like this, I would place a limit on the number of phones campers could use...and they'll just have to wait their turn. If, on the other hand, the camp is for rich spoiled kids, it doesn't seem like a big deal to factor into the price of the camp, the telecommunications cost for these campers, if it has to go that way. If so, I would go with a SIP provider as Steve suggests. But for security purposes, I would go with a PBX that has additional layers of security like call restrictions (or call blocking) and a per extension privilege restriction.