Aside from the many excellent suggestions about how to mange the devices themselves, I'd highly recommend putting a pihole on the whole wifi network.
I added a pihole to my home network via a <$100 NUC (very much overkill for this purpose), and the rate of my relatives falling for weird viruses and scams has gone way down.
I added a pihole to my home network via a <$100 NUC (very much overkill for this purpose), and the rate of my relatives falling for weird viruses and scams has gone way down.