I think that most of the Brompton purists would flog you for attempting to retrofit a kids front suspension to a Brommie, lol. In the words of Col. Kurtz, "The horror!"

Not to say it couldn't be done. If you want to keep the aesthetics of the brommie, there is a Pantour front hub that sort of dampens the ride. I've only read about it, but there is a elastomer in the hub that gives somewhat as you go over bumps. It either comes as a basic hub that you can lace to a wheel or a full on replacement wheel that you just swap on the brompton.