OP you have my sympathies 
My DS was 6 and in year 1 during the last lockdown. I am a key worker but was wfh so kept him home. My Nana died the day lockdown was announced (not covid) we were very close and I was absolutely devastated.
As a result,I did no school work with DS for the first 3 weeks 
I just couldn't function. When we did start,we didn't do loads as he wasn't that engaged either. He likes maths so we did that and I bought a CGP english book which he enjoyed. We read anyway but did little else. As a secondary teacher I really worried he'd be behind the others who were all posting masses of work.
He started Y2 in September and hasn't been behind at all, smashed the phonics screening in Dec as well.
Point is,your daughter will be fine honestly! She is so little still and it's probably quite confusing for her to do school work at home because that's not where she normally does that. Lego is an amazing toy -she will be using her imagination and developing her fine motor skills -great, let her keep going!
Try to weave 'learning' into other aspects of life. Can you maybe do some simple baking with her - let her help weigh things, look at the numbers together, count how many cakes you made, if we eat two,how many will be left etc. If you can get out - counting how many red cars pass, how many houses have a blue door etc. All helps.
Try to read but remember, you reading to her is massively valuable -just as much as her reading to you. You don't have to read school books only - I have read all sorts to my son. Gangsta granny was a favourite!
Easier said than done but try not to worry too much