Thought some of you may find this interesting and entertaining. I built a tandem for my four year old and six year old boys. I have a tandem that my wife and I enjoy and thought my boys would enjoy that same. Needless to say, they love it. I get a kick out of seeing how much fun they have riding it. I figure it would help reinforce working together as a team. They get noticed from both other little kids and parents seeing them ride it.
So far it's been a total blast. They learned much more quickly than I could have imagined that they can go much faster as a team. Before I was barely poking around to keep up, now I have to move it along and make an effort. Basically their top speed is limited to their pedal RPM as they can easily push that gear.
Having both boys on one bike has also been great at physically keeping them together. When they ride on their own bikes it sometimes becomes a challenge when one boy zooms ahead while the other is poking along behind. Do I speed up and leave the slow one behind or do I slow down loosing sight of the one in front. This is no longer an issue.
Here is a video of the process on how I made the bike.