Just idle musing but there are lots of people on both sides of the fence either keen for their children to return but they are in the wrong year or saying they absolutely won't send their eligible child back in the near future.
If the school has a set number of children they have deemed safe to have in the building, could they not ask all parents to complete a form stating whether or not they will send their dc in, then pick names at random/1st come 1st served from the yes list until the places are filled?
Rather than planning provisions for 60 year 6 children but maybe only getting 30?