@AymanRoshdy 

According to the Emulator for your router model on TP-Link's website the WAN DNS can be configured as follows.

tab "Advanced" -> "Network" -> "Internet" -> click the edit button (in the "Modify" column) for the WAN interface you want to configure -> unfold the "Advanced" settings -> check "Use the Following DNS Addresses: [ v ] Enable" -> entry fields for "Primary DNS" and "Secondary DNS" should appear -> enter the two DNS addresses you've mentioned -> Save

By the way, since you mention protection against malware and adult content. If the users of your network know how to manually configure DNS on their client devices (and set it to 8.8.8.8 for Google for example), then this would circumvent your measures.

@woozle Thanks for the detailed reply, much appreciated

Yes I understand that there is no 100% protection, I am trying my best here to protect my children from harmful things on the internet while still giving them the freedom of using it, if you recommend something else then I am all ears

Thanks again