It wasn't a choice between 'lockdown and remote learning' and 'normal school'.
During the 2-3 weeks before lockdown, absences among both pupils and staff were high. Some were ill enough to miss a week or more at a time. The pupils were incredibly keyed up and very worried about themselves, their parents, and the school staff.
If we'd continued without a lockdown, the weeks to the Easter holidays would have been utterly chaotic, and little learning would have been done. Pupils would have been turning up to classes missing a third to a half of their classmates, or more, with many lessons supervised by teachers covering outside their subject. As the number of teachers off sick increased, schools would have had to consider shutting year groups, because there were not enough adults in school to supervise the children. There would have been short-notice closures, with senior staff having to make the call on the morning, depending on who was well enough to come to school.
As this continued, learning would have become more and more fragmented and disrupted, with pupils coming back to school with ever-increasing gaps. In lessons, some children would have been floundering because of work missed, others bored because attempts were being made to go over work for those who'd missed it. And many lessons would still be being supervised rather than taught (no supply teachers available), with short-notice cancellations continuing.
If we hadn't stopped at this point, more adults would have caught it. At that point, lots of people were dying - around 4% if I remember correctly - lower amongst younger adults, higher amongst older adults. We've quite a few older staff, and some clinically vulnerable, and our parents skew older. So let's say we lost 2% of our staff (6 people), 1% of our parents (20 parents) and 0.1% of our children (1 pupil).
How would this scenario have been better for the children's mental health and well-being?