Agree with you all about young children and mental health.
My DS is 3. He spent the first 2 weeks of lockdown asking "what are we doing today?" (because we used to do something pretty much every day), asking to go to nursery, asking to go to his usual groups and activities, to go to the park, and to see his nana. Eventually he stopped asking which was a relief but also heartbreaking.
A couple of weeks ago he started getting very distressed at bedtime and waking in the night - saying he wanted us to stay with him, and saying he missed his nana and wanted to see her.
We weighed it up - decided it was low risk (mum is 59 with no health conditions, working from home and social distancing) and saw my mum.
Did us all the world of good.
The two things that would make a huge difference to children's wellbeing are being able to see a small number of other people - for us that's nana and one or two friends with children (as he's an only child for now) - and being able to go the playground! Those two things are really not much to ask!
I also think that many will benefit from going back to childcare/school, although I know this has to be managed carefully in terms of the extra demands on teachers and the reduced staff levels due to staff who are shielding or self-isolating.