Mini tandem I made for my two boys, 4 and 6 years old. My boys have long graduated from balance bikes and currently ride a 12" and 16" wheel sized bikes. My wife and I ride a tandem and I thought they would have fun on this. I also thought it wouldn't hurt to reinforce the concept for brothers to work together as a team. So far they make a great team and they are still brother.
The video show steps on assembling two 16" wheel size girls frames into one tandem frame. I also show how I coupled the timing and final drive chains along with the stoker handlebar mount I made. Girls bikes were more plentiful and in much better shape at the thrift store. The Y frame for the front triangle doesn't look girly and provides greater standover clearance making mounting and dismounting easier for the captain. The lower standover in the front translates to lower standover in the rear. The stoker rides a 12" wheeled bike so 16" is a bit of a stretch. It works very well for my boys. I finished the day before they attended the kids event at a local mountain bike race. Seeing them have fun every time they ride it has compensated me handsomely for what I considered a fun project.