Track my families total internet usage?

I'm a father of three and most of them are in their teens, this as i'm sure you are aware leads to excessive internet usage. My question to is how to monitor this, ideally I would like a program that will tell me how much data is being downloaded and to what platforms? ( computers, iphones ect.), because i'm sure that it is their ipods and iphones that are doing most of the downloading and/or streaming, please help me out because this is really racking up the bill as we have a capped data amount and it usually goes over it.

Download NetSpeedMonitor now! It's free! For windows XP/ 7 /8

you can trace the uses as well as download & upload speed

Please find the Link below

http://www.floriangilles.com/software/netspeedmonitor