Children living with parents in two households are able to visit both parents in both houses whilst self-isolating. I’ll find the part of the report ASAP but I had to help my friend with this about 6 months ago.
Edit 1: Here’s the link to the document with answers about this.
https://commonslibrary.parliament.uk/research-briefings/cbp-8901/
Edit 2: The specific guidance:
In response to the initial UK Government guidance issued in March 2020, the President of the Family Division of the High Court (England and Wales) noted that the guidance does not mean that children must move between homes, the decision being one for parents to take after assessing their circumstances (bold original):
Government guidance issued alongside the Stay at Home Rules on 23rd March deals specifically with child contact arrangements. It says:
“Where parents do not live in the same household, children under 18 can be moved between their parents’ homes.”
This establishes an exception to the mandatory ‘stay at home’ requirement; it does not, however, mean that children must be moved between homes. The decision whether a child is to move between parental homes is for the child’s parents to make after a sensible assessment of the circumstances, including the child’s present health, the risk of infection and the presence of any recognised vulnerable individuals in one household or the other.2
The guidance from the Family Division has not since been updated.
Guidance issued by the Governments of Scotland, Wales and Northern Ireland states that where parents do not live in the same household, children can be moved between their parents’ homes.