Is this a home network, business, ???
If this is a home network (or even a small office) most of your current routers have a parental mode/service built in which would do what you are asking.
If this is a traditional corporate network with a router and a firewall (two seperate devices) then check and see if the firewall has this built in. It may be an acutal hardware/module that you have to purchase or it could be an activation key that you purcahse that enables that feature.
If your firewall does not have this feature or ability then you can drop in a proxy server/appliance to monitor and track usage. Some will even do bandwidth quotas.
The above is just one example ... it would help to know the infrastructure a bit more...
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