European answer - I appreciate that some USA people are a bit more iffy about nudity. There's UK specific info as well - note that Brits are considered a bit prudish by European standards.
Here is the PEGI website guidance
https://pegi.info/what-do-the-labels-mean This is guidance on what is and is not acceptable for video games for the different age ratings. It's a system used in Europe (not sure if that's just the EU or the whole of Europe). PEGI 7 means not suitable for children under 7, etc.
The section on sex (i.e. what would require the sex warning label to be on the game) says "This content descriptor can accompany a PEGI 12 rating if the game includes sexual posturing or innuendo, a PEGI 16 rating if there is erotic nudity or sexual intercourse without visible genitals or a PEGI 18 rating if there is explicit sexual activity in the game.
Depictions of nudity in a non-sexual content do not require a specific age rating, and this descriptor would not be necessary." In other words, games with PEGI 3 (the lowest rating) can have non-sexual nudity.
Although this is for video games, parents' expectations of book content won't be that different. If anything, nudity's much less of an issue in books because you don't actually see anything. I can think of lots of British children's books with non-sexual nudity, including books for toddlers that involve learning about bath time and potty training.
Here's the British Board of Film Classification website's page about U certificate (universal/suitable for all)
https://www.bbfc.co.uk/what-classification/u - the section on nudity says "Occasional nudity, with no sexual context." - i.e. non sexual nudity is acceptable. In a context where everyone's naked because their culture doesn't do clothes, they'll likely shoot and edit the film carefully so it's clear that people are naked but genitals aren't shown on screen. (See the link at the end - but in a non-humorous setting the not showing of genitals would be a lot more subtle. Also, occasional glimpses aren't likely to be an issue.) ETA: they may not even edit it out if it's in the context of a documentary. I'm not 100% clear of the rules, but this is in relation to on-screen shots of genitals. A context where people just don't wear clothes is not an issue at all. Also, this is U certificate (suitable for all ages). MG books can have 12A certificate content so really, truly, what you're talking about is not an issue.
I realise that the USA is different (but surely not
that different?); there was a really interesting article about this on the BBFC website but I can't find it
USA parents tend to be more concerned about sex and nudity while UK parents are more concerned about violence and threat. Film ratings can reflect this, for example there are films that get a 15* rating in the UK because of threat/violence while in the USA they get a lower rating (like the USA equivalent of 12A*) and there's an example of a film that was PG* in the UK but got an adults only rating in the USA because there's a brief scene with full frontal nudity in it (can't remember if there was any implied sex or not, but there was no explicit sex in the film), but as it was not marketed as a children's film and the scene was brief it got a PG rating.
*15 = no-one under 15 is allowed into the film; 12A = children under 12 must be accompanied by an adult; PG = some scenes may be unsuitable for young children, but there are no restrictions on who's allowed in. Most kids' films are PG.
Also, last time I took my kids to the cinema, they showed this ad
https://www.youtube.com/watch?v=HgUzu1-g9-U and the film had a U rating. (Ads and trailers must be rated the same as the film.) I hope this allays your concerns, at least as far as European audiences go...