Exactly janipat. The government has been removing outdated versions of the rules over the last few days, but it's possible to retrieve them. I'm pretty sure it was permissible to leave home to provide childcare in an emergency, so the nieces or aunt could have travelled to London and picked up the child. However, in no circumstances should either parent have left the family home, if they thought they had been infected, and certainly not travelled as far as they did.
Apparently, Mrs C even took the child to a hospital, which was also forbidden. The advice was (and still is) to ring 111. There was the very real risk that she could have spread infection to patients and staff in the hospital.
Cummings has claimed that the child was "vulnerable" and needed "safeguarding". This is a cynical manipulation of an addition to valid reasons for leaving home (which was, I believe, added after all this anyway) to protect children suffering from real abuse.