I have a lot of miles with 2 kids on a trailer-cycle. A lower end model (no gears)...

Kids loved it.
It was safer than there own bike, especially in traffic. It did seem to teach them how to ride in traffic.

I saw the other day a guy with his 2 kids on his tandem. One on the stoker with a clip on crank! the other in a kids seat (fast asleep).

You can find pictures of tandems w/ trailers and or trailer-cycles in the touring with family realms.