I am using Asus 4G-AC53U router. When I turn ON "Parental Controls" to block a client's Internet access, does the router really block ALL kinds of Internet traffics including system's service (such as OS update checking, license checking, etc.) ? Or it only block normal user's access, e.g. web browsing, video streaming, etc.?
This small office/home router has a function named "parental control" which is actually a simple time-of-day restriction for devices. During the times of day that you configure, the specified device cannot access anything on the network at all.
It would depend on how it was determining what devices to block if it wasn't blocking all devices. MAC addresses can be spoofed, etc. If you are concerned about what your kids are looking at, a non-technical solution like putting computers with internet available in "family" parts of the house - living room, study, kitchen, etc. and not in a "private" space like a bedroom.