How to setup internet quota/access control

Hi there
I'm new to openwrt and replaced the system of my netgear r6800 with it. my goal is to control and manage access to the internet for my children.
I was able to configure the firewall in the way that access to the internet is shutdown to specific devices during a specified period of time.

--> can anybody help me to setup access-control to the internet by amount of bytes or minutes consumed?

Thanks a lot,

This might be of interest, especially the Traffic Shaping section.