> The iPhone comes with a web browser.

The iPhone comes with a web browser that has parental controls. A parent can have some degree of control over what their kids see when they use Safari. A parent does not have that choice with typical applications.

If that's the problem, then simply add a setting which apps can check.

If that's the concern, then Apple needs to deliver a parental controls API that apps can use to check whether or not the app needs to use filters. Otherwise, their complaints are wholly their fault for not having the functionality to let people discriminate between safe and unsafe content.

Sure they do. I’ve used those parental controls myself.

Every single app in the App Store has an age rating. An underaged user can only download apps in the age categories that their parents have specifically authorized.