What to do can be quite simple IMO
Dragging them out daily so they can get fresh air and exercise, making life as fun as possible, trying to be kind and patient in the family, eating well, having conversations, borrowed a dog, celebrated whatever was there to celebrate. Letting them see friends when it was possible.
But we had some very dark times (a combination of several stressful things happening at once during lockdown). One of my children said he felt suicidal for a bit because of all of it, but we bounced back from it by doing the above.
He is back to his happy old self again, sporty, sociable, doing well at school, mostly joyful.
My youngest sometimes says she wishes covid never happened and that since then she isn’t ‘creative’ or happy anymore. But she seems happy, she still wishes the carefree pre covid life back.
I think on the whole kids are able to bounce back, but it may need dedication of keep doing a lot of simple healthy things every day. And keeping an eye on it.