These days, that is generally done through an intermediary firm. I don't think many charities hire their own clipboard-wielding youths, so they wouldn't be counted as employees.

By the way, I had some trouble figuring out the right term to put into a search engine. Turns out the industry term for this is "face-to-face street fundraising". And in the UK it's sometimes called "charity mugging" or even "chugging", which you have to admit is more amusing than going straight to Godwin...