Standard measures for brake mounts?
 

Hola, new member here, building a custom frame for a kid with dwarfism.

Does anyone know offhand the appropriate location of the v-brake mounts when building a bike for 20" (406mm) wheels, i.e., appropriate distance from axle to mount?

Gracias!

While I don't have the dimension you ask for I would strongly suggest a trial mock up with the brakes and wheel in hand. Andy