itandem

ITchair is a child seat specific to Brompton folding bikes. It comes with foot pegs, but my son wanted to pedal. So I bought a Burley child stoker kit from Precision Tandems, supplied a few parts of my own, and made presumably the world’s first tandem that folds to carry-on luggage dimensions in seconds.

There are movies:

Took about an hour.
Footpegs removed, stoker cranks clamped on.
Folds fine this far. Further and you need to remove the ITchair completely (<1min), first by manually derailing the timing chain.