Some kids ones have woods valves not sure why. No reason not to ‘upgrade’ schrader. I suspect a brompton’s will be skinnier, aren’t they 16″ anyway? Generally a tubes a tube, especially at this end of the market.
Not all 20″ tyres/tubes are the same 20″, will depend on if the width has a decimal point or a fraction in it’s measurement, as to what it’s ERTRO number(and thus fit’s fitting size)will be.A 20×1.5 will be impossible to fit if the original was 20×11/2 as far as tyres go, though you may be able to force a tube to fit.