How to prevent the child from deleting the app?

You can prevent the child from deleting the app by disabling the iPad's home button. This can be done by using Apple's Guided Access feature, which disables a portion of the screen, as specified by you. 

Tap here for more details : How to prevent the child from exiting the app?

For iPads and iPhones:

There is a setting that you can set up to prevent app deletions: 

How to prevent app deletion on iPads and iPhones with Screen Time?