Also, the rules are in place to protect people. Travelling to another town puts people in that town, and on the route, at risk. It is certain that at some point when Cummings had the virus he will have risked exposing his children to it, and under those circumstances asking his parents to care for them puts those people at risk, too. The question of whether or not they were sick at the time is irrelevant.
I'm sorry, but if you think it is a good idea to break rules that you have written, then that makes you a hypocrite, and it makes you someone who will abuse your privilege to do something you expect others not to. The Government created these rules to try and protect its citizens. We were, in your own words, "duty bound" to follow those rules. That goes doubly so for the people responsible for creating them. There is nothing exceptional here, and the rules even covered this, as has been pointed out several times: if you had symptoms of Covid-19, you were supposed to stay in isolation for seven days, and people in your household for 14.