Switch the default browser to Firefox and change the default search engine too. Firefox will protect them from all the third-party, cross-site tracking. It's faster too.
And if this causes an issue for something in the next day or two, you're still there to help them out plus the previous default browser is still there too.
But they both used Chrome. Today I switched them over to Firefox with uBlock Origin. And with some wankery you can make firefox look like a reasonable Macintosh application.
https://i.imgur.com/VaMnux3.png
But so far so good. And, as I have commented in the past about the pihole and when people leave the wifi at home they worried their phones were compromised because ad people have lost their fucking minds.
So now the twins can can have a reasonable internet on their computers.
Please can you un-ignore that in one sentence? ;-)
On the other hand, too much frustration is just too much, so you definitely need to balance.
Ublock has broken all kinds of sites for me, and I don't know if it's because these sites used a third party JavaScript library or some video provider or what, but I know to turn off all the privacy tracking things I have in my browser when a webpage looks broken, and often that fixes it.