I tried mounting the baby seat on the trail a bike, but then my 18 month old pulled the 4 year old hair the whole time. It was great fun for the 18 month old (boy), not so fun for my 4 year old (girl) I think this is the start or many years of "little" brother teasing "big" sis. The look on his face was one of just pure joy as my daughter screamed!
Anyway, I think this (
http://bikebalance.com/images/attach_ChldStLG.jpg) would work as it would still give a "neck" for the trail a bike to attach too. But I can't seem to get ahold of anyone at the company and everything is out of stock.
Then this is interesting, but I can't find a US distributor, or a kids bike without brakes.
http://www.followme-tandem.com/english/index.htm
This is my fav option, but again no US distributor that I can come up with. It is actually designed to use with a child seat
http://www.bikesandtrailers.com/trai..._add_bike.html