DURHAM, N.C. — Family vacations typically take the Kennards to Lake Cumberland, which is technically the ninth-largest reservoir in the United States, a halcyon spot that’s a four-hour drive from home in Franklin, Ohio. At the three-bedroom getaway house about a mile from the lake’s south shore, ensconced in the quiet of South Central Kentucky, everyone can boat or ride four-wheelers or just put the phone aside and, even when the extended clan is on hand, dissolve into the stillness. Last summer, though, there was an urge to take a separate trip with a smaller cast, a nuclear family-only excursion.