Instead of giving them less karma (and having weak/bare-bones characters), I would give them the same amount of Karma, and either 1) have some additional restrictions such as lower Availability or maximum Magic of 4, or 2) since they are all on the same page, just let them have free reign, since they want to make low-powered characters anyways.
Look at some of the SR5 archetypes such as the bounty hunter, face, or sprawl ganger - you don't need less points to have a street-level game, just less of a focus on optimization.