I don't know what they should have done legally @TheNumberfaker. But let's just analyse what they did do and how they could have done better.
Wakefield starts throwing up with coronavirus, Cummings comes home. Fine. 24 hours later he becomes ill to the point of going to bed, with a fever. They decide they'd be better off at his parents because they can isolate AND have someone on hand if the worst happens. So they drive 4-5 hours, presumably stopping for a pee at least once. I don't know many 4 year olds that don't need a wee in that length of journey. One of them would have gone with him. So you now have at least one symptomatic Covid 19 patient wandering into a motorway service station loo.
Alternatively, if grandparents had agreed to keep an eye on the child, one of them could have come down by car or train and stayed with them or in a hotel. As I said, at his level, if he'd have called Downing Street, they probably would have helped facilitate it as it was helping a person who was sick, so allowed.
But you don't drive all those miles if you're so ill that, by day 6, your partner believes you need to be in hospital - which is what Wakefield wrote in her Spectator article that I linked to above.
The well travel to the sick not the other way round.