Doing a very rough calculation, about 1.8% of the UK's population is employed by the NHS. So if you say that roughly 1.8% of parents are healthcare workers, you could then open schools/provision with a skeleton staff for those pupils?
If parents are mostly able to work from home, there's less need to send children to grandparents. However, people who rely on grandparents for childcare will probably send them anyway in the holidays- those kids are at less risk of passing something on if they haven't been in school.
FWIW, I'm still not sure if it's the right time to close all schools yet- but I'm not convinced those are arguments to keep them open.