When our kids were younger, we bought some really nice walkie-talkies for them. (Make sure you get at least the 5-mile radius, preferably longer. Cheap 1 to 3 mile ones don't work onboard ships due to all the metal and electronics.) Plus, they work great in port and a whole lot cheaper than paying roaming fees with cell phones.
Travel Agent/Cruise Specialist w/12 yrs exp and 47 Cruises on 11 cruise lines! Favorites: Paul Gauguin - Tahiti: Uniworld River Cruises - Europe; Celebrity Solstice-class ships; Holland America - 12-nights Baltics & Russia; RCCL - 14-nights Greek Isles, Turkey, & Croatia; Holland America - 14-day Alaskan cruisetour; 10-night Canada/New England cruise; 21 days Hawaii w/7-night NCL cruise; Oceania - 25 days in Asia; more than 3 months touring Europe by train. And many all-inclusive resorts!