Depending on your family dynamic the monthly data limits on the phone should be all you need.
Allocate them what you want on the phone (e.g. 5Gb each with a warning to appear at 4Gb usage). AS @SamSam says they can easily circumvent these but the point is they'd have to do so wilfully and in a way that you can check their usage towards the end of the month and see they've decided to take more than allowed.
They can use wifi to minimise SIM data usage when available (I've recently put in a couple of google wifi units which allow you to allocate wifi to particular devices in home so wifi on/off at the touch of a button whenever I want and auto off after 9pm for the kids devices etc.)
You can decide if you are happy to up their allocation if they hit their limit in any given month taking into account how much data you have left as a family. You could even ask them to contribute a bit of pocket money towards any extra data requests (Optus charge $10 per Gb but perhaps $2 mates rates? No one values what they get for free.)
Peter Gillespie
PS If you have Apple Phones then unfortunately like Optus, Apple don't believe in enabling you to control your data usage effectively. 
