I finally found something after much google searching. I have several kids scooters and balance bicycles with small tires I just can't get a pump onto. However, I never found a clamp on type, just screw in (which is awkward if you just want to fill and go).
The extenders seem to be sold mostly for truck and motorcycle users. Good keywords for this adapter seem to be "valve stem extender" or "flexible Schrader valve stem extender" or even "right angle valve extender".
There is a Chinese company that offers flexible Schrader extenders at:
http://www.wheel-weight.com/Tire-val...nsion-c20.html
And a UK vendor sells them at retail:
Schrader Valve Pump Connector
Metal right-angle versions are sold in the USA at:
But the best option seemed to be the Topeak adapter, which has an internal valve to reduce pressure loss on removal (making it easier to unscrew as well):