I don't know. As has already been pointed out, "pocket money" is Brit English to the extent that it would likely get a completely blank stare from 95% of the 315 million denizens of the USA. This is not a reason not to teach it, of course, but is a reason to pause before teaching it as a "handy idiomatic phrase."
If I were teaching a course in (primarily) British English, though, I wouldn't hesitate to teach it for that reason alone. I would, however, hesitate to teach it as an example of "the kind of thing one is not likely to ask more than once in a human lifetime." I base this on the fact that I, as a native speaker, have never in my entire life, child or middle-aged adult, asked that particular question of anyone. Therefore, the question of whether to use the simple short form of "How much" + noun, or the much more natural-sounding "How much do you get?" is rendered immaterial. I call this kind of thing ENPP: English for No Particular Purpose.
