I would have thought that one of the type which attach to the seatpost would work ok - although the child might look a little odd suspended in space between the rear wheels rather than hovering a few inches over the back wheel of a bike as is normally the case. From an engineering point I don't think there would be a significant difference. A neighbour has the christiana I think (big wooden box up front?) and he puts 3 children 6,4 and 2 in there - might be more difficult to get them all on your trike.