I'm using a Chariot hitch on my Brompton towed Carry Freedom City . trailer.
the hitch elastomer is a ball end.. So, the Chariot kid trailer with that hitch ..
with a Brompton, the rear fold self stand is unimpeded
because of the ball easily rotating in the hitch, fixed under the left axle nut...
details of hitch
Thule Chariot Carrier Parts | Bike Kid Shop | Bike Kid
you can cruise the site for the actual 1 & 2 child sized trailers..
a bike seat wont be so useful after they get bigger , like a trailer will be, long after they are pedaling their own bike..
hauling groceries to feed yourselves for example..
Otherwise
There is a Spanish site selling the second saddle mount, to fit in front of you
http://www.milianparts.com/en/products/pere/
also this
http://www.milianparts.com/en/#!/BRY...tegory=4418112
to atapt to fit one of these
http://www.milianparts.com/en/#!/BRY...tegory=4418112
it will all be hanging off the handlebar mast. turning with the wheel