I take my kids on an older Bike Friday tandem. I believe the current models use more standard stoker stems, but mine is a 2 piece system that uses a standard quill stem as one of the parts, which allows for some adjustability. It has been sufficient to use with the stock handlebars since my kids were about 4 or so. I don't know whether such a thing is available for purchase, but I believe there adjustable stoker stems out there.