Not sure if anyone has said about this app, but there's a great app by none other than Google called Datally which allows you to control which apps you want to use your 3G/4G and also setup when you want data on and off during the night and setup your data allowance on a daily basis.
I'm use the £20 Goodybag and worked out I can use around 1.3Gb a day which I've got setup on the app, then it warns you when you are getting near your personal allowance.
I know there are other apps in the Playstore, but many either are riddled with ads, or require you to pay for them after a while.
Hope this helps some users who like myself have demanding kids who stream while on the bus!!