I would take a look at crank shortners. Depending on the size of the bike and size of your 6 year old they might work. I have a set and with them I can get my 5 year grandson on two of my bikes. Not all seats and seat posts are the same height from the top of the seat tube. I found a seat post that let the seat set lower and a thinner seat and was able to fit him on the bike when he was 4 with the crank shortners. If this works for you they are fast to take on and off when you switch stokers and will save you some money. TandemsEast has them.