My children carry my wife's phone while they are out (and away from our wifi at home) and before you know it, BOOM my data plan is used up, this month in 6 days into the cycle! How can I set her iPhone 5c so that it will not allow data usage when not connected to wifi? Even if there was a way to password protect so that data could not be used unless a password is entered would help. There already is a password for the phone. But the kids know it and use data. Any help will be appreciated.
Open settings app and disable cellular data.
You can set up a second passcode for cellular data restrictions. See this site or apple support or ask a new question if you need assistance with that optional step.
I would use the moment to teach the kids about responsibility. Let me choose if they should know the passcode and be responsible for the costs.