I stumbled across Watsonians last night while researching.
Watsonian Sidecar Images - Gentleman Cyclist | Google Groups
The design is incredible simple. I have some ideas about how to design the pivot connection, and i figure once the frame is done make a cool looking box to put on top of it, and stick my kid in it.
I still have concerns about connecting to the chain stay, I figure I have about a five inch span between the crank and rear derailer. Will that five inches be enough to over come the torque of the cars want to pivot on the axle, and will that cause undo weight/stress on the bike? I will find out soon enough when I make it, but I was hoping that an engineer here could quiet my fears.