The deradicalization of my parents' YouTube is working. @Jairus only had to get rid of a couple recos here and there, a gentle prune. It's taken a few visits to get here, but now we're seeing real progress.
The process: go through recommended videos until you sees something sketch. Go into that video's extended options, select "I'm not interested" or "I don't like this video" (different on mobile vs tv). You can also select "don't recommend this channel" to get rid of whole channels at once.