If the device is new and has been left on or with apps running constantly in the back ground.. this will drain your data very fast and I was going to say lucky you because I've yet to use my 2GB data simply because our cell signal isn't strong enough to draw data from the system.
Also this would depend on the type of plan you have.. If you're near a wifi/modem terminal/signal the data should not be used because wifi (assuming this has been installed in your home) is free or that you've been paying for the service within your home used for the Internet etc.
Typically a drain like this is to due to not having or being located near a wifi terminal. Instead it utilizes the cell signal which draws or drains the data. I would suggest that you systematically go though both phones and localize/turn off all none essential apps or use a app minimizer like Opera max. A simply drag and drop app which will scale back and save data drains like this for you.
I'm under the impression that children being children tend to leave apps running 24/7 thus the extended data drain occurs.