You can't the idea is that you unlock the device and hand it to your kid. The solution for the time being is Apps Corner, which is available currently in available GDR1 (Update 1). To get this update you will need to enroll onto the developer preview program via a free app studio account, as it is not out for the general public. I would suggest you back up all your files, pictures, videos on your PC / laptop before doing so.
One thing to bear in mind, the photo hub is accessible via the customize option in the kids corner.
However once you have enrolled and updated to GDR1, should you encounter any bugs or problems you will not be able to revert back without using the Nokia Recovery Tool. Which means you will lose app data. game saves and anything you haven't backed up in the process, therefore you enroll at your own risk.