You mentioned they aren't very tech savvy. Given this, DO they even need internet? What are they going to use internet for? Will everyone in the party be in close proximity for the duration of the trip?

WiFi is easily available these days in most areas, often free. There are plenty of apps with offline maps and directories.

If you're worried about battery drain, get a battery pack for the primary phone. It'll be cheaper than lighting up all of the phones with international roaming, and you shouldn't be traveling without one to begin with.