For a trip of that length, I would skip Pamukkale. It's quite out of the way and, at least in my opinion, a B list rather than an A list site. I'd think instead about adding Ephesus. You can fly cheaply directly from Istanbul to Izmir via one of the discount Turkish airlines--Onur Air or Pegasus Air are fine, and then return to Istanbul and fly to Cappadocia from there. If you wanted, you could then go by bus to Ankara, spend a day there touring the world class Museum of Anatolian Culture, the impressive Ataturk Mausoluem and museum of modern Turkish history, and take the all night train back to Istanbul--a little bit of Orient Express atmosphere? Or just fly back out of Ankara.
If your budget will permit, a hot air balloon flight in Cappadocia is unforgettable. My husband and I didn't do it our first time in Cappadocia, but we did the second time and it is one of our all-time travel highlights anywhere in the world.
Kids can do the Turkish bath, but I wonder if the intimacy will be uncomfortable for girls that age. Even though you can be partly dressed in undies during the bath, kids that age are often hyper self-conscious about their bodies. Also, other women bathing may be totally nude while they are there. Make sure they know what to expect--someone actively scrubbing them all over--and that they are enthusiastic about trying it. Otherwise, I'd skip it.
I wouldn't worry about safety--it's no more or less safe than anywhere else in Europe. My daughter traveled in Turkey as a late teenager and got pestered more by would-be rug sellers than would-be dates. Obviously, young teenaged girls need to be sensible about their conversations with strange men no matter where in the world they are, sadly enough. But, assuming they are not planning on rambling through Turkey on their own, you need have no fears.
