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London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
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Fishfabs
These kids bikes have a mechanism that you might find interesting:
https://blackmountain.bike/products/skog
(Basically, the bike is expandable and when you make the frame bigger you also take off an outer part of the sprocket to make the gear bigger, then to tension the belt and extend the reach, the seat tube rotates a bit to move the bottom bracket forwards and the the seat back.)