This depends on the bike. On our PeriScope 26” models, a child as small as 3’ 6” can ride without any special equipment. On a more traditional tandem frame, children as young as 2 1/2 have been started tandeming with child stoker kits.
Yes, child stoker kits are available to fit on virtually any tandem. Here’s a photo of our Co-Motion child stoker kit:
The geometry or size of the frame is not important when these kits are used. Typically, the tandem is fitted to the parents, and adaptations are made for the the child. Essentially the kit allows for the attachment of a child sized crank-set at any point on the stoker seat tube of any tandem. A chain connects this small crank-set to the non-drive side of the main crank-set. The drawback to the child stoker kit is that it must be removed when adults wish to ride together. Another adaptive device is called a crank shorten-er (below).
This device shortens the radius of the circle that is pedaled by the child, and also reduces the distance form the saddle to the pedal when the pedal is at the bottom of its stroke. Crank shorten-ers can be used to extend the age- or size-range of a traditional tandem, even when the real issue isn’t the full pedaling circle, since they can allow a smaller child to reach the pedal at an earlier age than they would reach the pedal at the crank’s full length.
Because of the dual telescoping seat post system and low, compact frames on our PeriScope models, switching stokers between an adult and child is very easy. Once the child is big enough to ride without crank shorten-ers, between 5 and 7 years of age, you can change the set up a Periscope for an adult or child stoker in a few seconds, as it’s simply a matter of adjusting seat height and handlebar height and reach. Here’s a photo of the PeriScope system:
More info on the PeriScope line of tandems is available here!