adams makes a tandem trail-a-bike:
http://www.trail-a-bike.com/product/...iginal-tandem/
They used to make a chair-back kind of add on for smaller kids to use until they grow into the regular bike seat, but they seem to have scaled it up to a full on child seat for up to 40#:
http://www.trail-a-bike.com/product/...ies/baby-seat/
that doesn't look designed to allow pedaling as best I can tell.