Kinetics is for the front only? Or do they assume you have your own rear triange for rear discs too?
Re: the baby seat, I mean like a simple horizontal bar with foot grips that goes on the Brompton 'top' tube. Pretty sure I've seen these, unless they're some custom jobby.
Bang for buck, which set up is best if I was to go down converting my existing Brompton to a disc brake one?
https://www.kinetics-online.co.uk/folding-bikes/brompton/brompton-disc-brakes/