Safari will automatically display a warning message if you try to visit a site that is suspected to contain malware or phishing. This feature will alert you to sites that may be dangerous to your device, but not to sites that may contain inappropriate or illegal content. 

Remind your child that they should never share personal information on sites/services with people they do not know, and this includes usernames and passwords. If a site requests their personal information and they are not sure if it is okay to enter it, encourage them to check with an adult first.