Several companies sell something similar for "adaptive cycling". BuddyBike sells this one which is sturdy and reasonably inexpensive at $70.

http://buddybike.com/Products.html

For even more support - Brown Cycles sells a seat post attachment to hold a child seat (similar to a car seat), a toddler seat, or a highback recumbent seat. They are more expensive, but if you need something like them then they are well worth it (the quality is good too).

http://www.browncycles.com/tandems.htm