How to restrict adult content on UIWebview?

问题

I am working on an app which is specifically designed for kids.So we are giving some search option for the kids through google search engine and making it sure that search should be safe which we are doing by adding &safe=activate in the url.

This is working fine and the search content is restricted.Now user enters another search engine in google say bing and open it on UIWebview. So like google it is also restricting the adult content but asking permission of age limit.If kid agrees that he is 18+ age then he can browse adult content using bing.

I want to make app in such a way that it should never show adult content on google search on UIWebview.

I have the following doubts.

How would we verify the url loading on UIWebview in shouldStartLoadWithRequest is an adult website.I tried to change the url (by adding &safe=activate every time when user loads subsequent UIWebview pages) but it is assigned readonly property and we can't change it.So is there anyway to control there?
Should I make a list of adult websites in my app and check every time when url is loaded in shouldStartLoadWithRequest.
Is there any possible way to restrict search engines like bing not to ask kid about age permission.