I did it with my grandmother’s Facebook a few years back by blocking most of the worst of the pages that were spewing that crap. Her feed got boring for her and she switched to reading erotica

I would doubt the efficacy of watching stuff on their account to try to shift the algorithm. For one, the algorithm appears to naturally select for controversial (and more likely right-leaning) content on its own when left unchecked and without significant history to the contrary. Secondly - and this is the one I can’t help with - your parents are selecting that content. The root problem is there. You can watch all the left-leaning stuff you want but they’re going to counteract it and the algorithm is going to back that up. To the algorithm, what’s more important, a small and recent interest in content entirely unrelated to what it’s accustomed to, or hundreds of hours of watch time on accounts the user is still subscribed to and also watches?