When your daughters are at home, are their iPhones plugged into external power?
If not, then you may have a case which you neglected to ask teh Apple rep about. When an iPhone goes into a "sleep mode" the screen will turn off, and the wifi radio will also turn off. If your daughters are using their phones for streaming audio, then the phone screen turns off, and the phone goes into the sleep mode, it will turn off the wifi radio (to save battery) and your iPhone will revert to using cellular data, even though you are well within range of a valid wifi network.
This issue has been discussed many times in the Apple support forums, and it is mentioned here on the AT&T support forums as well. There have been users who have tested this using a wifi analyzer which shows the cell phone wifi connection dropping after a period of time, but the cell phone continues to stream data but now using cellular data instead of wifi.
One way around this is to connect the iPhone to anexternal power source when you are in the wifi area. I use my iPhone heavily, and I also use external power much of the time. Because of this, I generally use about 1GB of cellular data a month. My wife, on the other hand doesn't use external power as much, and she uses over 5GB of cellular data each month, even though I use more overall data.
One last comment I will make is that when the iPhone 5 was first released about 2 years ago,mine would still turn off the wifi even if it was connected to external power. This would cause my iPhone to use more and more cellular data because it was not usig the wifi when I wanted it to. I don't remember which iOS update corrected this for me, but it was corrected in one of the many iOS updates.
