For kids of the age described, this is what I see some Dutch parents using. One kid gets to pedal, another seat can be bolted to the top tube between the pilot and his/her steering column. 2nd kid gets footpegs. The configuration can be modified. When older kid wants her own bike, younger one moves froward into pedalling position. When the next kid moves on, eliminate the front pedalling position and bolt a plywood box to the rig to haul bags of RediMix.

I see some parents with kids in position 1 and 2, and another one in a child's seat behind the captain. Looks very Dr. Seuss-ish.