Fber - it's not "the nanny state" - it's that cultural norms have changed, people don't want children seeing adults smoking. It's not that the bulk of society have no problems with children seeing adults smoking and just someone in whitehall thought they'd like to stop it, it's that the bulk of parents do care.
As I said, the social norms around smoking has changed over the last 20 years, it's gone from being ok to smoke at your desk in many offices, to having smoking rooms, to no smoking indoors, to smoking being something the bulk of society don't do anymore.
It's now around only 20% of the population smoke, it used to be more than half, it's a big change in a generation, and it's mostly poorer people who smoke, so if your family is many middle class professionals, the rates of smokers they are around is likely to be a lot less than 20%.
For a lot of people, it's not normal part of life anymore. Thinking about accomodating smokers isn't something a lot of non-smokers have to do - and besides, most smokers are aware they are in the minority, are at a non-smokers house and if they really couldn't wait, would ask .
And it is bad manners to smoke around other people's children. Most people don't want their children to think of smoking as a normal acceptable thing to do. So seeing a relative smoke would have more impact than someone on the street.