Anything that consumes a lot of data would be best left to the bonus zone. Things like software updates, for instance, or downloading large files, like a movie.  

I would suggest installing the free Glasswire software to see what's using the data. The software only measures data usage from each device it's installed on, and it's available for Windows and Android.  There are routers you can get that measure usage for the whole network, so one of those may be useful, though they are expensive. 

If you have 10GB per month for the household, this means that the household can use no more than about 1/3 of a GB per day in a 30-day month, a bit less in a 31-day month. That's a very small amount and would likely hold if you only limited yourselves to browsing and email.  If you stream anything, that would eat your data very quickly, and you'd have to seriously budget for it by not using the data for much else.  A movie in HD can consume 3GB per hour. You'd be out of data in less than 4 hours. 

You'd also need to lock everything down to prevent automatic updates, and if you're using anything like a Roku or a smart TV, for example, you'd have to disconnect it from the wifi, as they use data even when you're not actively using them.