It's stupid, but it's 6 in a house. if it was 6 in a "gathering" then people would be twisting the rules to have 6 in the kitchen, 6 in the garden etc. So it's a simple, blanket rule of 6.
It's illogical and stupid, but that's the law.
Er... I don't want to be rude but have you actually read the relevant legislation? You seem quite happy to boldly state what it says, but what you are saying is blatantly wrong. It is the "6 people in a gathering" that the law bans.
The bits of the legislation that are relevant are:
During the emergency period, no person may participate in a gathering which consists of more than six people unless [certain exemptions apply]
And
there is a gathering when two or more people are present together in the same place in order to engage in any form of social interaction with each other, or to undertake any other activity with each other
It would be far too great a stretch for the courts to interpret sleeping children as being within the definition of gathering.
I do think a court could potentially interpret the definition of gathering broadly to cover multiple groups meeting within the same property. The individuals in such a scenario would be in the same place in order to engage in social interaction with others, and arguably there's no requirement that every single member of the gathering should be interacting with every other member of the gathering. In which case, the 6 in the kitchen, 6 in the dining room, 6 in the hall example would not be legal. It is an entirely different kettle of fish to a situation where you do not have more than 6 people socialising because the 7th person at the property is unconscious upstairs.