My DC are now 16 and 12 and have grown up entirely in Germany. Yes, in many ways it's way healthier than the UK. Some of the neurotic things I read about on here are absolutely bonkers by comparison. On the other hand, the UK is in some respects socially more progressive than Germany, which can sometimes appear to be stuck back in the 1950s. And we live in the capital city, which is notoriously 'liberal' by comparison with the more conservative rural areas!
Your points in detail:
Young DCs walking to school alone in cities: depends what you mean by 'young', and across the city? In villages it's traditional that kids go alone from the time they start school at 6 or 7. In fact, older people particualry seem quite offended at the 'over-protected' kids these days whose parents take them to school. In a city like this, many kids will still be accompanied until they're 8 or 9 because the traffic dangerous. DD1 first went on a public bus by herself at 8. DD2 went on a bus with DD1 at 7. But they didn't really get used to travelling across our large city until 11 - which is more or less when they would in the UK.
-Pre teens being allowed to travel across the city on trams: see previous point. DD1 and 2 started going across the city at 11, including a very busy train intersection. By 12 or 13 they're pretty competent to get round by themselves. DD1 first flew by herself at 15 and is pretty good at that now at 16. There is a gradual progress towards teenagers taking more responsibility and getting more independent.
-DCs being naked on beaches: er yes, of course. Why not. Pre-schoolers have no problem with this. There are also a number of FKK beaches (nudist) and saunas are compulsory naked. The restriction on children in saunas is because of health concerns (children and hot temperatures) rather than nudity.
-DCs being allowed to use knives: yup, DD2 was bought a child's woodcarving knife for her 7th birthday (from schoolfriends, not us!). Never been a problem with it.
-kindergartens having sleepovers from age 3: absolutely, never been a problem. In fact, it might well have been 2 when they started. DD1 went on a 5-day trip with her nursery when she was 4, and I know of children who went earlier than that. Again, no problems.
-schools doing frequent week long trips in primary: tends to be 5 or 6 days rather than a whole week, but yes, from year 4 onwards every two years or so. In fact, they're regarded as a compulsory part of the curriculum - families who can't afford the cost can get it subsidised by social services/welfare payments, because it's regarded as important for social bonding with the group. In my DDs' school they went abroad in year 6, but not every school does that.
-1-5 year olds in kindergarten focussing on social skills and play: yes, in fact that's falling out of fashion a bit now, and nurseries are starting to at least introduce the alphabet to pre-schoolers. The idea is to create the social and motor skills which will prepare them for learning and a class setting. And you know what - both my DC learned to read two years after they would have done in the UK, yet they learned to read in two languages incredibly quickly. By the age of 8 or 9 they had absolutely caught up with the UK.
-not learning to read until 6: see above.
-DCs being allowed out to play to build dens in woods: yes, why not. But then, I grew up in the UK in the 1970s in the countryside and that's what we did too. So it seems to me that what the UK is doing these days is very much a step backwards - restricting children while purporting to be protecting them.
-12 yes being allowed mopeds: eh? Where did you get that from? It's 16 for a moped, 17 for a car, I believe.
-good manners being expected eg no screens at the dinner table I don't think Germany is the only place that doesn't tolerate children using screens at the table. There IS a particular intolerance to burping by children, however.
"Best of all German kids do better than British ones in international tests in their teens."
Where on earth did you get this from? The Germans are convinced that they do awfully at international tests. There was a notorious test in 1999 called PISA (run by OECD) when German did terribly, coming behind the Czech Republic, for instance. It was called the 'Pisa shock' and it led to a huge crisis in confidence in education. Many Germans believe places like Finland are much better. Please don't take Germany as a good model for education! They select kids into three different schools after 4 school years, and parental background plays a huge role - and there seems very little interest in making things fairer. The education system seems prehistoric by comparison with the UK, where there genuinely seems to be an effort to help kids to achieve as much as they can. Education in Germany is positively Darwinian - if you can't hack it (even for non-educational reasons), you're thrown out to the next level down.