I am curious about your experience at Therme-Erding. I am planning on going as well-my daughter will celebrate her birthday while we are there and I thought it would be a perfect way to spend part of her day.
Therme Erding is amazing. I spent five hours there on my last full day in Germany, recharging physically and mentally for the trip home. It's a great place to relax and unwind. English-language website is here.
(Note that the "Sauna-Paradies" section of the facility is for age 16 and up only. Swimwear is not allowed in this area, though everyone covers up with towel or robe unless actually in the pool or sauna. There is only one (huge) coed locker room. But this is Germany, where it's no big deal and there is no hint of any impropriety.)
Take the S-2 to the Altenerding stop (not Erding, which is the end of the line). From there it's about a 3/4 mile walk on a paved path, through a construction site, to the side of the Therme Erding complex. Or take a bus from the S-Bahn station to the Therme.
There is an affiliated, similar but smaller facility at Bad Wörishofen.