Very flat land and young kids? Single speed. Because they're gonna abuse the bike, not using kickstand, dropping it on both sides banging the pedals on concrete, I had to replace a neighbor's kid's stamped BB cup because it fractured that way.
Very flat land and adults? Stylish light or retro beach cruiser single speed also works, dead simple. Super bonus points for belt drive on a lightweight frame and wheels, easy and clean carry on shoulder.
Slight hills and want to be forced to push hard out of the saddle for training? Sure.
Slight or steeper hills and want to pedal sitting down? No, it damages your knees. Never lug while sitting.