The spa was absolutely wonderful!!! I could easily spend an entire week there. On the Summit (and I believe all m-class ships), the T-pool was free. It was comfortably warm (not hot). I believe it was a mineral/salt-water. It had lots of jets and was absolutely wonderful. There were also a couple of hot tubs in the aquaspa area.
The T-pool and its associated hot tubs are in an enclosed, nicely warm area, with windows all around. The Aquaspa Cafe is also in this area, and the food was out of this world. We ate lunch in there twice; what a treat!
Just off the T-pool area are dressing rooms, showers, and with a dry heat sauna. These are gender-specific and free of charge. The dry heat sauna has a large window - what a place to enjoy sail-away from port after active shore excursions!
There are separate charges for other spa services - massages, Persian Gardens (a group of co-ed aroma therapy type steam rooms that are simply glorious), etc.
The T-pool is limited to adults after a certain time. During the day, children are permitted only with adult supervision. We only saw a handful of children in the T-pool area, and they were all supervised and well-behaved. I never saw any children in the Persion Gardens or other areas of the spa. Because we had late seating for dinner, we did not use the spa at night, so I don't know if the evening age restriction was enforced or not.
Hope this helps. Enjoy your cruise!