This is about the total number of social interaction each household has and trying to reduce them. Each time a single person meets a single other person thats an interaction and that increases your exposure risk as a household as transmission once in a household is difficult to stop.
This also means that in lots of cases, babies and toddlers WILL be having their own unique interactions if they are cared for by people outside your household.
Therefore thats why they are included in The Six in England.
By the same token, I can see the argument that if you have a very young baby that is cared for exclusively within your household and therefore has only exactly the same contacts as you (so can't be a fresh out of hospital newborn in case they've had any tests etc when you weren't there), you are only very minimally increasing the risk (higher risk is baby could catch something even if you don't at the same interaction).
However it remains the case that legally speaking if you have a family of 4 which includes a newborn but you meet with another family of 3, then yes you are breaking the law.
I guess it depends on whether you feel willing to take that risk. If the other family tests positive and you are flagged up on track and trace you might have an issue - and thats really where you are most likely to fall foul of the law, rather than be reported by a neighbour with twitchy curtains.
The behavioural experts will factor in a certain level of adherence to the rules - they won't expect 100% complience. But at the same time enough people need to accept social responsibility and suck up the law for it to work too. If every found loopholes and excuses to allow 7 rather than 6 people you are multipling the risk 6 times by every single social interaction that extra individual has had.
Each extra person has a whole chain of contacts that are multipled by every person they meet. It is important that number is kept as low as possible.
It sucks but it does need high rates of compliance to work especially since children at school are having such a high number of potential chains of contact they are being exposed to.