Brompton is absolutely a game changer for multimodal transportation if your not taking kids.

My wife and I both have bromptons, and I recently got this very secure, but inexpensive seat post child carrier that works with the brompton. The only problem is it screws on and I cannot lower my seat with it, so it interferes with thr fold a bit. I'm going to experiment with quick release seat post clamps on it, which I think should be quite secure (same company has a similar seat post, it just comes with QR seatpost clamps and is less available and more expensive)