If you are worried or concerned what is using your current data plan there is an easy way to search/track/control this.
The place to look for this on your iOS device is Settings->Cellular.
If you scroll down you will see the section "USE CELLULAR DATA FOR:"
Under here you can see all the applications that use mobile data and how much has been used since the last time the counters were reset or from day one of you using your phone.
If your kids or even you have been killing your data plan then you may turn off certain applications so they will only work on wifi, like YouTube or Netflix or even some games that download data/updates as you play.
When you look through the list you may be surprised at how much data some applications may have used. If you do want to track it for a month then I recommend you clear the counters by pressing Reset Statistics right at the bottom, then you can have a starting point.
I am always interested in feedback so please feel free to add any comments, or you can mail me here. If you would like to submit a quick tip with full credit and links back to your site then also feel free to contact me.