Open dns is a free online service that you can use with your router at home to restrict access to inapropriate contents on the net.
It block access to porn website, gambling and many more sick web sites that can be detrimental for childrens.
It works by forcing those dns into the dns settings for all computers on your home network.
Primary DNS : 188.8.131.52
Secondary DNS : 184.108.40.206
You can set is up by changing the default dns on your router or into the compuper directly (for more granular control.)
You can set it on the children computer only if you don't want these restrictions on all computers at home.
Below is a screenshot of a typical router.