For VM's web filtering, the devices need to point at the VM DNS. By default most wireless devices use their software writer's DNS (so Amazon, Google etc).
There may be web filtering options in your Amazon account settings, or you can change the DNS entries to 194.168.4.100 and 194.168.8.100 (VM's DNS) and see if that gets the intended outcome? If you're dealing with teenagers rather than younger children then be aware that they'll soon learn how to circumvent DNS level filtering......
I'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media. Learn more
Have I helped? Click Mark as Helpful Answer or use Kudos to say thanks