Password lock on the iPhone is the first step, next step is to use Parental Restrictions (Settings>General>Restrictions) to prevent 'find my iPhone' being disabled. There's a step-by-step
here and if you scroll down through the comments there are also options mentioned for replacing the simple four-digit passcode for a full alphanumeric-any-length-you-like password.
Sorry, just edited to say that the full alphanumeric password seems to only be an option for the iPhone lock screen, not the Parental Restrictions.
HTH Pete