It's been my experience that as people get older they tend to become set in their ways. ( And no disrespect meant to any forum members because I'm 66 and in that "older" category.
Every so often various magazines and independent companies publish their findings as to which AV is "the best". If you search around I'm sure you'll find some comparisons that rate Avast or MSE equal to or better than Norton. Using those comparisons and perhaps throwing in the argument that free can be as good as or better than a paid product, perhaps your folks will change their minds.
But as seavixen said, it's their house so you gotta play by their rules.