They are, however, quite likely to take it home and give it to Grandma, particularly now going and visiting her indoors (albeit not hugging her) is allowed from 4th July (while distancing is still stated, I think grandparent childcare is now de-facto back, as there is no way to enforce otherwise). And then it would be a problem. So we do need to try, within reason, to prevent infection.
Kids have lived, and still do live, in far worse settings and situations than we are in now. They are a lot more resilient to adversity than people on here seem to think - indeed, much, much more so than most adults. All they really need (other than food, water and shelter, obviously) is to be loved and to know they are loved - that sounds trite, but it's also very true. An acrimonious divorce, for example, will do a kid far more damage than the present situation because it calls that basic need into question.