To NOT know what their children are up to online.
Get a Facebook account, get a Twitter account, get a TikTok account.
Tell your kids they aren't allowed their phones / WiFi passwords if they block /
Restrict you following them...
If I'm wrong, then I'm wrong... happy to admit it if I am.
But being 'too old' is not an excuse, my 75 year old father can learn new technology and has so much more knowledge than me. It's not an excuse. (Except medical reasons - including mental disabilities / health)