Harry van der Liende has produced a whole series of boxes that were designed to transport two kids on a trike. Both white and black boxes have been built.

year:	?
place:	?
maker:	Harry van der Liende
source:	Photo collection of Johan Vrielink

Willem Dijkstra made a trike rear part to be able to transport his two kids, as well as a reasonable amount of luggage.
You can read more about it (text only in dutch but with illustrating pictures that you will understand) at his site.

year:	winter 1998/'99
place:	Hellevoetsluis (NL)
maker:	Willem Dijkstra
source:	Photograph by Willem Dijkstra

Cock van Driel built this version of the kids' reikshaw. Also for two children. He has built the self supporting plastic (styrofoam with polyester coating) rear part himself. (Note: The builder seems to have ignored the risk of kid's fingers between the spokes!)

year:	?
place:	?
maker:	Cock van Driel
source:	Photograph by Erik Wannee This trike is also described in HPV Nieuws no. 2, 2002 (kids special), page 16