Twenty Twelve

I don’t have an iPhone so I can’t test this, but you could try changing:

From:
<meta name="viewport" content="width=device-width" />

To:
<meta name="viewport" content="width=device-width, initial-scale=1" />

It’s usually located in the header.php file.

If your child theme doesn’t have a header.php file then simply copy and paste the parent theme’s header.php file into the child theme’s folder and change it there.

You can also check out these links for more info:

https://developers.google.com/speed/docs/insights/ConfigureViewport
https://developers.google.com/web/fundamentals/layouts/rwd-fundamentals/set-the-viewport?hl=en

Ah that worked thank you! Could you tell me what that line means / why it fixed the problem. – just for learning’s sake.