A brilliant spa with many facilities, including pools for children, quiet areas, and the sauna (adults only).
I would recommend a full day visit to Therme Wien, for the complete experience. The sauna area is separate and changed in addition, which wasn't made clear. However, it is well worth the money! There are a multitude of rooms to visit, and a couple of lovely outdoor pools, as well as ample sun loungers to relax on. The sauna area is of course a clothes free zone, which we were comfortable with. We would highly recommend it, after a few minutes it all seems very natural.
The only downside to note is the queue to get in. We had pre-booked and it still took an hour to get admitted to the spa.
Book yourself a cabin too rather than a locker for a couple of Euros extra, worth it. Go to Therme Wien you won't regret it!