I used to do tech support for Bitdefender and I can say that outside of a handful of people from our team, we actually didn’t care for it as an antivirus. The free version in itself was more or less didn’t really do much anyway, at least at the time.

You can still use it for them, but I find that Windows Defender does a good enough job on its own - it’s also what I set up for my mom (almost 70) on her laptop + a very basic crash course of what to avoid (don’t click on links from strangers, if you get a message like “Is this you? The person in this video looks like you” don’t even look at it, and so on) and other basic internet safety tips.

Just in case, I’d maybe recommend setting up sometime (maybe once a month?) where you just run a scan on their computers to make sure everything’s alright; I’d recommend Malwarebytes for it, I remember it was what we used for remote interventions too, actually. I’m not sure if it’s doable by yourself, but maybe you can rope someone else into helping you with it.

Worst case scenario, you can always just set up parental blocls for them, actually. Set up what they can and can’t access and so on.