A routine Monday to Friday of getting up on time, dressed, breakfast, sticking to a timetable of homeschooling including breaks, reasonable lunch including fruit or vegetables, finish with homeschooling at aset time even if not everything is done.
Outside every day no matter what the weather, for at least a full hour.
Family dinner around the table.
Screen time limits but we're pretty generous around other activities - set in stone is no individual screens after 7pm (no switch, no phones for teens unless they have a readon, but tbh my teens are absolutely fine with no phones after 7pm and my 9 year old doesn't have a phone), no PC gaming, no consoles but family TV together is fine.
Set in stone school night bedtime, read aloud to younger ones then they read if they want. Later, slightly more flexible weekend routine but still always dressed before lunch, always an hour outside, always family dinner at the table.
Chores - sounds counter intuitive but you need something to rebel against and something to be glad to have got over with and a contrast to appreciate - ours do the dishwasher and bins on rotation and put away their own laundry, and are responsible for all guinepig care including cleaning out (with prompting if necessary but no physical help).
I think mine are helped by having one another, even if they squabble a bit that's also normal social interaction, and they do overall mostly get on. They're also helped by the fact they've always lived in the middle of nowhere and been mostly responsible for entertaining themselves. We saw their grandparents (in laws) a lot until MIL died 4 years ago but sadly without MIL FIL isn't that interested and they'd got used to rarely seeing him, and we only see my parents once per year as we live a flight away. So not much to miss...
The most important thing is to acknowledge any down or angry or frustrated feelings as real and vslid but not indulge too much and remind them that in objective reality things are not that bad - being bored isn't so terrible, and its not forever.
The worst thing to do is indulge in catastrophising - they take most of their cues from their parents.
In all honesty lockdown isn't that different to how my childhood was in long school holidays until I was in my mid teens when I could get about more on my own. A bit dull. Make your own entertainment. Go for bike rides. Write letters (I had to ask to use the landline and my mum listened in, so my friends and I wrote to one another...)
Mine are mostly pretty content tbh, though occassionally one has a blip, and we listen, sympathise but if they get into a spiral of working themselves up become a bit no nonsense and remind them its temporary and the same for everyone and really just being bored in a warm, dry house full of toys and books and tech and cosy places to sit isn't as bad as what a lot of people have to deal with.