It's not going to be compulsory for children to go- no action will be taken against parents for non -attendance of their children.
From they way some teaching unions and teachers are talking you'd think this had been sprung on them but in fact schools I know were asked about 8 weeks ago to draw up plans and school governors I know have been finalising plans for ages based on new classroom layouts, smaller classes, one way systems, staggered start times and breaks.
If we don't try some sort of phased return now when will we start one? September? January? If we wait until September would that have to be a phased restart spread over maybe 4 months? Far better to pilot it now and see what issues arise so that the plans can be tweaked if necessary for the Autumn term.
I do think that if the alternative was laying teachers off or furloughing them at 80% of pay (likely to fall to say 60%) it might focus the minds of the unions. I don't think it would be unreasonable to say that if schools are to remain shut until September to satisfy teaching unions and councils (apart from the current key worker/vulnerable children cover) then the government funding for education would be cut accordingly for this year by a modest say 10% to reflect the closures.
I would make one change, however, to the plans- whilst i can see why Year 6 would restart I am not sure of the need for opening up for Reception AND Year 1- I suggest just one or the other initially.