A bit late posting this update but wanted to pass along a very nice family friendly facility we found while traveling in Germany late summer 2024.  After touring nearby Heidelberg (wonderful castle tour and hiking area), on a Tuesday evening we went to Miramar therme.  https://miramar-bad.de/

On Tuesday evenings at a specific time (varies in the summer based on school vacation season I believe) the full facility becomes naturist. Bathing suits prohibited. We arrived about one hour before to have an opportunity to grab a bite to eat and get the lay of the land due to large size of the therme.  The naturist time was prominently posted and announcements were made about 15 minutes prior.  I would say 60% or so of the people left as the naturist hour approached.  But for the three or four hours of naturist time, I would estimate there were easily still 200 to 250 people at the facility (it is so large it is very hard to estimate). There were easily two dozen families of various sizes.

Our kids had a blast on the 6 different water slides, the wave pool, and the beach/lake area. Everything seemed very safe. Good lifeguard presence. On these Tuesday evenings the entire facility is naturist not just the sauna area.

As you will see on their webpage, sadly there was a recent fire and the facility is closed until the Spring of 2025, but you can use the link above to check for updates.

So far I have not found other thermes with facility-wide, family friendly naturist hours but if others find some do feel free to share.

yes, Miramar is a great place. Sadly that they have changed the nudist times to 8 a clock in the evening on long saturdays and tuesdays at 7 o´clock in the evening. One hour less - but better than nothing.