@ascensyon 

If the MAC address of the wireless card of the desktop PC has changed from what it was at the time when you added it to the parental controls profile, then it will have no more effect.
To verify this theory you would need to compare the current MAC address of the PC to the MAC address stored in the parental controls profile. However, the way TP-Link has laid out the parental controls setup page, it seems it's not possible to review the MAC address that's stored inside the profile. (unless I've missed something)

Anyway, if the above was the case, then the only way to prevent this would be to create a Whitelist (under Security -> Access Control) of all legitimate client devices. That way, if any client device changed its MAC address it would not be able to connect to the network at all.
 

@woozle thank you for your response,  but the live mac address is the same that's saved within the parental control profile.

i've even tried to black list the mac and it still went through.  seems the only way was to reboot the router and then it took again.  but it dont want ot have to reboot the router each time.

@ascensyon 

If it works fine initially after a reboot, but stops working after some time, then it could be a flaw in the router's software. That would be something for TP-Link to investigate.