We did the JMC trip to Pumi. (edited to say FROM Kusidasi which may not have been obvious)
Basically there is a big hill/mountain range and a small part of it has the white calcium deposits which are left by the water.
Hotels etc took all of the water and so the white became grey prompting the ousting of the hotels. The deposits were slowly turning white again when we were there.
One old motel houses Cleaopatras Pool which is supposedly in the same state is has been in for hundreds of years. (break your leg on the debris if you are unlucky!) The water is very warm and you are supposed to imbibe it as it cascades into the pool for maximum beneficiary effect
There are a couple of ways into the pool, one life threatening to non swimmers so make sure you go the right way. There are free changing rooms.
Make sure you have enough money to go in the pool as it is not included in the journey and it is not loose change so its a long way to go and miss out.
On the approach there is the ruins of Hierapolis but our trip did not have time to pause as we had spent time at Aphrodisias which has a very impressive stadium, and even more time at a Diana hostelry for lunch (drinks (water) not included). We only had time to wander down a couple of ledges and then jump in the pool to get wet before we had to get in the coach to get back.
The trips run by the people in the town do slightly different things but we started at a godforsaken hour in the morning and got back just in time for last meal at 9, for less than 1.5 hours at the site.
The Kusi waterparks can take up a couple of days! You don't appreciate them really until you have tried somewhere else (Benidorm) we did aquafantasy and adaland. They were different and both worth visiting. They are both on the same dolmus run.
So is Ephesus. If you take the kids on a DIY via the dolmus (Selcuk route) and walking they will have the freedom to explore without the constraints of having to follow a tour guide like sheep. They may then actually enjoy it.