A child stoker kit allows a child to pedal a tandem. The kit is a crankset that bolts to the stoker seat downtube. It has very short 130mm kid-length cranks. (A typical adult crank is 170-175mm). Since it bolts to the downtube it dramatically shortens the distance from the seat to the pedals and thus lends itself to the vertically challenged.
The circular pedal motion is carried down to the main stoker chainring via an extra chain. This chain drives an extra chainring on the inside of the main stoker crank. In other words, there are two chainrings on the left side stoker crank. The outer one is connected (through a chain) to the captain's crankset, the inner chainring connects to the child stoker kit.
Santana claims 3 1/2. I laughed at their marketing. Anything to sell "quality time" to their customers. But sure enough at 3 1/2 my daughter wanted to ride on the back. It helped that she had seen our son on the tandem (he's 4 years older).
A 3 1/2 year old looks pretty small sitting way up there, and her interest in holding on with both hands does not always meet parental minimum standards. But the only problem we have had is a couple of times her foot has slipped out under hard acceleration. Fortunately I've stopped pedaling fast enough so that she did not get hurt as she fell forward. I do recommend toe clips and cinching them down.
1) You need the child stoker kit which includes the crankset that bolts to the seat tub, plus an extra chainring on the inside of the left stoker crank, plus a short chain to connect the two new chainrings. And you need a set of pedals.
2) You need a longer handlebar stem as a standard adult one probably has the handlebars too close to the captain's seat.
3) You have a long seat post in the stoker position. It should be on the order of 12" in length.
In practice you don't have much leeway in where you mount your stoker kit. For really small children you may have to move it toward the top, but for most, position the stoker axle so that the crank just clears the front derailleur.
The child stoker chain runs on the inside of the adult stoker crank so comes quite close to the left chainstay. On my Visa the chain barely clears the chainstay. On my Arriva, no problem. Even if there is a clearance problem, you may still be able to adjust both the front and rear bottom brackets to move the chainrings far enough out so that an inner ring can fit and give you chain clearance with the stays.
You need a longer handlebar stem. For a 4-7 year old I used a very long unit (almost 9-inch) plus a set of bars that are twisted in the up position. It looks weird but your child will not want to ride in a dropped handlebar position. The bars are narrow so as the child grows knee clearance becomes an issue. But if you buy them to an adult width and your child is small, they have to spread their arms considerably to reach the 'grab on' parts.
1) Move the pedals down. The child cranks (without pedals) look funny and somewhat imposing whirling around, but they don't seem to interfere with an adult cyclist.
2) I recommend an entire adult handlebar/stem/captain's seat assembly to swap in place of the child handlebar/stem/captain's seat assembly. Otherwise you, at minimum, need a handlebar/stem combination to slip on the existing seat post.
The entire drive train is one system. You may have to adjust everything in your drive train to fit a stoker kit. (Positioning the stoker kit may require the main stoker chainring to be moved causing the front chainring to be moved, causing the front derailleur to be moved....). Nothing impossible but very easily time consuming.
The problem with the stoker kits that I have is that the stoker can only be lowered to the point where the right side child crank starts to interfere with the derailleur. That means for my setup, the total amount of up/down child crankset movement possible is only about 4 inches.
Probably somebody has done this successfully but the clearances were too tight for me to want to work it out. So to answer the question, since you can't lower the stoker kit you have to keep raising the seat post as the child grows. And sooner or later you have an 80 or 90-LB weight on top of the rear seat raised to it's maximum which makes for unfavorable weight distribution.
How long you can use the child kit depends on the size of the frame and the size of the child. My son is 9, 56" with a 24" inseam sitting on the back of a 20 1/2 frame (measured top of rear top tube to center of crank) and he appears ready to move to the crank shorteners. He's tall for his age so maybe your child could keep riding until they were 11 or so.
Crank shorteners bolt to the adult cranks but give you three or four additional holes to attach the pedals e.g. giving you effective crank lengths of approx. 100mm, 120mm, 140mm and 150mm.
The problem here is that your child's heel tends to interfere with rear panniers. I like Dick Powell's (owner of Bicycle Outfitter) suggestion for touring with a B.O.B. trailer and intend on trying it. The lack of the combined weight of front and rear racks plus the panniers helps offset much of the weight of the trailer.
My personal testimonial is that it really is a great way to spend time with the kids. Sure it can be a bit of a strain when you are dying climbing a hill trying to get enough oxygen and the little being behind you is rattling along about what he or she just saw on the road, but once you get past the idea that the child is not on there to help cycle but to provide you with company, it's easier to accept. And I positively feel lonely riding my single now. Someday my children are going to want to ride their own bikes (well they already want to do that- but someday they will be capable of covering the miles) and I am not looking forward to cycling alone again.
Start them early. If you wait until the child is 7 or 8, and is a stubborn kid, you may have trouble getting their buy-in.
(A minor aside- we put the cyclometer on the stoker handlebars and that makes for some child involvement. I've added a connector so that when I change to adult, I can unplug the unit and drop in new handlebars.)