I can endorse the kid-back stoker setups. My eldest girl has been stoking for over a year now, and just turned 5. It's noticeable extra power, not a lot, but it makes her part of the team and not the luggage.
My younger daughter rode on the seat that goes over the rear wheel, on the rack. That worked pretty well and everyone enjoyed it, although if she kicked her feet loose they could hit the older girl's feet.
I can't speak to the wee-ride from experience. But when we had our third child I tried to visualize one in front of my child-stoker, and didn't see how it could fit as the 5-yold's arms aren't long enough to reach around properly.
Maybe your 6-year old could do so, and maybe different handlebars would make a difference.