*I find what you write interesting. I'm not sure if you intended your aggressive tone?
Given that I SPECIFICALLY wrote about private saunas, in family homes, it's interesting that you jumped from my post straight into an anecdote about public saunas being problematic.
In the context of this thread and beyond, I think that "prudishness" can often be an innocent-seeming cover for actually quite repressive, obsessive, shaming and ultimately damaging thinking, where normal human bodies are coupled with sexual perversion and things which are taboo.
And looking at this from a population level, that damaging "prudishness" might possibly be part of why so many men in this country are susceptible to following online rabbit-holes that lead to CSA content... Maybe they're following the thrill of the taboo, that starts with something as normal as naked boobs, and ends up with terrible violence and harm?
It's my hunch that if it were more normal to hang out with naked friends, mums, aunties and grannies, and to find naked human bodies no big deal, then possibly we'd be in a much healthier place, sexually, as a culture.*
I don't feel aggressive about it. But I do bump up against the insidious shaming and chilling effect that language like 'prudishness' and 'repression' is intended to have on people who are asking totally reasonable questions about where the line should be when adults are getting naked with children.
I used a public sauna as an example in my last post. But here's a private one for you; a good friend of mine at university grew up with nudist parents - one of whom wasn't his biological parent. It utterly fucked him up for life. We lost touch a few years ago but last I heard of him, he was still in therapy and had just broken off his third engagement because of his inability to have a healthy relationship due to sex addiction.
Granted that's only one piece of anec-data but it does challenge your assertion that de-sexualised nakedness as a part of daily domestic life would prevent later perversion or issues.
Personally, I think that liberalism (and 'cultural norms') can often be a cover for at best, inappropriate and at worst, criminal behaviour.
My children are 8 and 10 and already instinctively want privacy from me, and also give me privacy. If they walk into my room as I'm dressing, they walk back out again and ask when they can come in. I haven't enforced this, it's something they have just started doing themselves. (I actually rarely close bedroom or bathroom doors in our house because we have Siamese cats and they complain loudly whenever they can't get in!)
Natural shame is a protective emotion. It helps keep us safe. It is too often confused with toxic shame. But they are not the same. One is necessary, the other is corrosive.
Subverting the instinct to feel natural shame in service to liberalism can be very uncomfortable, and even traumatising. I can imagine the dissonance experienced by a child wanting to put their own boundaries around their privacy but who also wants to please their parents by taking part in 'family nudity', for example.
All discourse around this topic on this thread has focussed entirely on what the adults feel they would be okay with (or think they ought to be okay with). Bolstered by odd and unsubstantiated arguments that it's 'healthy' or even necessary to know what adult naked bodies look like. Given that every single body is completely unique, I'm not sure what general conclusions are supposed to be drawn from knowing that mum's labia minora stick out and auntie Janet's don't.
I'm afraid I simply don't believe that children want to be naked with their parents - or any other adults, for that matter - after the age of about 8 or 9.
Speaking plainly, I would definitely have concerns about a family who regularly got naked together with children beyond pre-teen age. And I think most other people would too, if they're honest. Whether you're from Berlin, Malmö, or Stoke-on-Trent.
In the OP's situation, I think her mother is being very inappropriate not to consider how her grandchildren might feel about seeing her in just a pair of bikini knickers.
I get that this might make you defensive, because you're probably quite emotionally invested in the idea of co-opting some sort of quirky, novel bit of Scandinavian 'culture' to scaffold your idea of yourself as progressive and position yourself in opposition to uptight Britishness.
But I simply don't see it the same way. Perhaps I'm too cynical. Or maybe I'm just realistic.