How do I set site restrictions on iPhone?
From here

General

Once you are in the settings, go directly to the general tab.

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Restrictions

Tap the restrictions tab and once on that screen choose enable restrictions.

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You will now be asked to enter a 4 digit passcode. Do not forget this passcode, there is no way to retrieve it, and a full restore (re-installation) of the iPhone is needed to reset it.
Restriction Options

After you have entered your passcode, you will now have the ability to restrict any of the functions you see on the screen. Without going through each and every option, lets just take a look at Safari. If you turn off Safari and head back to your home screen you will see that the icon is removed and there is no way possible to reach that application any more.

To re-enable a function like Safari, return to the restriction page, authorize with the 4 digit passcode you entered above, and turn it back on.