The guidance given by Boris in March was specifically that children should not be left with elderly grandparents who are in the vulnerable category even without having symptoms. The stated exception was children moving between separated parents.
Cummings has symptoms, and took a huge risk in that his kids are likely to have spread his symptoms on by then.
If Cummings was well enough to (presumably) drive from London to Durham and back, he was well enough to look after his kids. Lots of people have no choice but to look after their kids when ill.
He will argue he had no other option for childcare reasons and say his parents property was the most effective place for his family to isolate. If he and his wife became too sick to look after their child then the grandparents could step in, although not ideal it was his only solution and the best option for the child’s welfare.
No fan of Cummins but I think the fact this story hasn’t surfaced for 6 weeks shows how weak it is.