There's an advanced "Touch" setting that gives you another shut down option independent of the power button.
It shut down the phone. I did a soft reset on start-up and it seemed to fix the problem.
We'll see if it lasts.
I suspect the daughter never turns off the phone, so it may have developed a software glitch that needed re-setting.
Non functioning power button again led to inability to power down Iphone. It was talken back to Apple and replaced without any problems. They recomended setting it up as a new phone rather retsoring under the old back up.
Of course...my daughter ignored them so she wouldn't lose all her contacts. So far, it's working fine and hope it stays that way.