Yep, what Maillotpois said, child stoker kit. Mine's from Chuck's bikes, not as nice as some of them, but it fit our budget much better and does the job. I got the shortest crank arms they had with his scrawny little legs being so short and all.
Today we went for a ride, 70 degrees and 15+ mph wind. We started out with bike shorts and jersey, biking gloves, fleece sweatpants, long-sleeve shirt, and light jacket. About 8 miles in he was cold and wanted his winter coat on top of everything else. That kept him comfortable for the last 18 miles.
I almost didn't bring the coat but knowing him I tied it onto the camelback. Glad I did. 
Nancy, your kids are the coolest. And here I'm hoping to work up to RAGBRAI in the next few years! Have to say, I'm in awe!
I used to have an open mind but my brains kept falling out.