Chewbacca and Summerlove, I feel exactly the same reading these posts.
I spent the day looking after youngest dgd (2.4 years) today, but at their house rather than at mine for once.
Both DS and DiL have had to work from home since March, and were on conference calls almost the entire day. If they weren't able to do this, they would quite literally lose their jobs, and then eventually their home, so it's not as if they have a realistic alternative.
They are lucky, because I am able to help out, but not all parents have any help whatsoever, so is it any surprise really that their children regress, when their parents are not in a position to monitor/deal with their toilet needs etc at the drop of a hat?
In our case, lockdown and no nursery has meant that between us we have actually been able to toilet train dgd sooner than would otherwise have been the case, but we are all very aware that things would have been very very different if they didn't have grandparental back-up!