I just had a look at your account and it looks like you switched off 'permit app installs'. I have enabled it for you so please restart the device and it should pop back up.
App Store App isn’t available on my Dauughter's phone after I installed the mobile zone app . How do I set it up so that she can have a game on his phone downloaded from the App Store
It sounds like the Permit App Installs setting has been changed to not allowed. You can borrow the device to yourself to install an app using the Mobile Zone app and your Parent Pin. You can do this by tapping the name circle in the Mobile Zone app.