Haha I expecting this question to have a simple answer. Since I'm not 100% sure what the correct answer is I'll ask my parents to ask which line to get on or if they feel more comfortable to just go to the visitors line at the SFO airport.
In case it helps, it says at https://travel.state.gov/content/travel/en/us-visas/immigrate/the-immigrant-visa-process/step-10-prepare-for-the-interview/step-12-after-the-interview.html that "When you are admitted, you will enter as a Lawful Permanent Resident, also called a green card holder" which to me implies they should go to the LPR line but maybe I'm not reading it correctly.