I think there's an element of the sunk cost fallacy here. I don't say this to berate you (covid has done funny things to all of us). But recognising it might help you to feel less anxious.
You were absolutely right to keep your kids home in March, as we were told to do, and when much less about the illness was known. (It was known from the beginning that kids were less affected, but I think how powerfully age-stratified this virus actually is has only become apparent over time. Not sure if there is another like it.) However, it can't have escaped your notice that a lot of people have returned essentially to normal life. You could have taken your kids on a summer holiday, on lots of playdates, swimming, to see their relatives, for meals in restaurants, to the bookshops to choose new books, to the climbing frame in the mark every day. Etc.
If you didn't do these things because you were anxious about the virus, it's possible that you simply made a bad parenting choice and deprived your children unnecessarily of fun and stimulating things this summer as well as (more important in my view) social relations outside the home.
This is a sunk cost. You've done that. We are all very prone to rationalising our sunk costs as the right choice, so that we can justify them.
Here it might be time to let it go. There has not been any reason for people without serious health conditions or children/ family with similar to be unduly anxious about this virus for months. It's fine to go back to school.
The rational things to worry about are a) the long-term effect on your children of losing six months of normal life and education, b) any non-covid health problems they might have that you've been putting off getting checked out (hearing tests, skin problems, digestive issues, anything...), c) your family's financial position.
Rather than making the first experience of going out in the world being school (I used to dread it for all the normal reasons), why not make the next week all about all the fun you can have with them in the last week of summer? Definitely make sure they see their friends; going back to school if the other children have all been meeting up all summer will make them feel left out.