We just got a second seat for our Brompton! This one is much larger and excellent quality with 5 point harness, reclining seat, and head rest with space for helmet.
I picked up the seat second hand, brand new, from someone who couldn't fit it onto his Dahon. It's tough with foldies, but a few modifications (mainly to the setback distance to the seatpost) and we were in business out riding in East Coast Park that very same evening!
This video is the very first time that Ethan and Talia rode on the Brompton at the same time. Listen for squeals of joy! It makes it all worthwhile...
All three of us loved the experience of being together. Ethan and Talia are quite attached to one another, so this new experience of being with one another and with Daddy is quite a thrill. Of course the speed makes it all the much better, though needless to say since I was carrying an additional 35kg load it was slower than usual.
More recently, I took both of them out on a more adventurous urban outing. We did some short distances on roads from Pinnacle to Lau Pa Sat for a satay dinner, then on to Marina Bay Sands for the evening light show. It was a great success, especially since I had tweaked the setup and the tire pressure so we were much faster. It's just amazing how well the Brompton handled this load. As someone once wrote, "A folding bike with small wheels like this shouldn't be able to carry loads this well!" But it does... As we were eating satay, Talia looked over at our folded Brompton with two child seats contorted around it and proudly said, "That's our bike Dad!"
Tech note: Total weight < 110kg, front bracket = 15kg, rear seat 20kg with mounting point < 5cm to seat clamp minimizing moment loading. I wouldn't ride the bike daily like this, but for the occasional outing it seems well within Brompton tolerances.