He should have been self isolating. Even if he or his wife didn't have it, Johnson did, and he'd been in Johnson's company.
If he needed assistance with childcare, then it certainly wasn't right to engage his (70yo+) parents to do that.
Whether or not he stayed in a separate building at his parents' property is irrelevant. He took his child, who had been exposed to his own/his wife's infection/symptoms, and then exposed his parents to potential infection (either directly, or indirectly, through his child).
Maybe what he did had the blessing of his parents. Maybe he had no other option. Maybe no further infection has resulted from his little trip .......... but he broke the guidelines (as they were at the time), and yes, it was wrong.
Should he resign? Based on the precedent of Calderwood and Ferguson, I suppose the answer is "yes" .................. but in reality, it will be up to his conscience ........ so he probably won't ................. and there's no way on earth Johnson is going to sack him.
Taking back control.
Edit - just reading this morning that his sister might have been at his parents' house also, and it was her who was looking after his kid (as reported by Kuenssberg). How convenient. It's frankly quite pathetic that since the story broke, all we've heard from are "sources", If it's all so innocent, why not just clear things up?