Tbh I am more surprised that DC got caught doing this, I thought he was smarter than this.
At the end of the day, a lot of people have broken lock down rules, I bet you most of the people on this thread have but won't admit it on here. I think DC reasons were valid, he knew that him and his wife could get seriously ill and it is possible that they don't have anyone they trust nearer by to look after the kid (just because you have relatives that live nearby it doesn't mean they can step in to look after your kids, I couldn't ask my inlaws for instance). If I only had one family member I trusted with my kid I would have done the same.
However, he's a very prominent political figure and he knows that the left hate him because of Brexit and the recent election result, so he was very foolish in doing this in a not particularly discreet way.