I have child stoker kits setup on my triplet and tandem. Both are Santana-branded, made out of blocks of aluminum with nylon spacers. I first had a model (Burley, maybe?) that was made of steel and did not have the nylon shim. I like the models that use the nylon insert better, as it seems to distribute the stress better and also is less likely to scratch the frame.

On a triplet it's a bit of a pain to set up, but it's pretty easy on a tandem.

To answer your questions:

1. Is there anything I need to look out for as far as fit goes other than tube diameter and bolt circle on my cranks?
> Tube diameter is key, of course. See my thoughts on using a nylon spacer with the BB adapter. BCD on your cranks isn't important. You can use a 110mm BCD stokid crank with a 130mm main drive crank, for example. The important thing is to match chainring sizes between the stokid crank and the drive crank so things don't get out of whack. (Of course, you'll need a chainring with the correct BCD for your main drive crank.)
2. Do all the child stoker kits have multiple pedals holes so the effective length on the cranks is adjustable?
> No. Most are set at ~125mm. DaVanci does make and sell a nice set of modified cranks that have two sets of holes at around 125 and 140mm.
3. What would the proper length be for a 6 year old who is about 4ft tall?
> My 6 year old son who is about that height does fine with the 125mm crnaks
4. Does anyone know where to buy one and not have to spend 260.00? I have seen them on ebay and craigslist dirt cheap before I needed one, but they are nowhere to be found when I am ready to buy. At 260.00 we are approaching the "I can make one myself" range.
> Just saw one on Ebay today. I found both of mine used through the Forum or the Tandem@Hobbes list (can't recall) and from a biking friend. Even used they hold their value pretty well. I paid $125 for one set and $165 for the other (had more components with it). Good news is, if you buy used you won't lose much value when you resell.
5. Do you pull it off when your full grown stoker rides or just remove the pedals?
> I've tried 1) just removing the pedals, and 2) pulling the cranks. Adult stoker still found the BB intrusive when riding. I really don't want to have to remove the BB and chain each time an adult rides, though. I haven't had to worry about it yet this season.

As Phantoj mentioned, the handlebars can be tricky, too. I used a cheap, standard 25.4mm seatpost (cut off the head), which is inserted into my stock stoker stem "receptacle" (bottom half) using a 25.4-29.8 shim to make up the difference. Then, a cheap 1" threadless stem clamps onto the seatpost-turned-stem (the 25.4mm seatpost "mast" is equal to 1"). A small, narrow handlebar is then used. Going this route was pretty cheap, as I had a set of handlebars that worked, and I bought the seatpost to cut down ($14), the shim, and the stem ($15) from my LBS.

E-mail me if you want to see some pics.