I guess so, if that's how it works. It will be pretty disruptive for working parents of younger children. I don't know the answer, but as a grandparent I think many will seriously have to consider helping out with childcare.
The government have had their heads in the sand for months about this. It's why they should have worked with local health authorities and got an efficient test, track and isolate system in place. It would have, in my opinion, been better to aim for a blended face to face/online offering, which would have made social distancing possible.
It's quite likely that a child will be sent home form school because somebody in the class has tested positive. The child will need to be tested and the results awaited. Until it's known for a certainty that the child is negative, anybody would be at risk of catching an infection from the child. A grandparent, unless living with the family, would be an extra risk. It's also possible that the grandparent him/herself would be at greater risk. Many parents don't have family to help out anyway. The risk might be low, but it will happen. That's what I meant by Russian roulette.