Many Android phones have the ability to set a data limit in settings. However it is easy to remove as well.
One parent to another...
Set the data alert on your account to 50%. ( I think you can set more than one alert)
If you get a text alert before the month is half over, login and see who is the data hog. Warn said data hog to use wifi or lose data for the rest of the month.
I did it to my son, he was miserable. But he never did it again.