I've been thinking about this for a while. So is there only one grocery store in the area or something? Could you possibly change your shopping schedule to a different day or time of day?

If that doesn't work. You have known these people for 20 years now... you should have a pretty good idea of how they act and think. Take a page from The Rehearsal and hire a couple actors to play your parents. Next you run through different dialogue trees (with the actors) until you figure out the correct responses to get your desired outcome.