How can I make this app in the phone invisible or protect from removal?

It is impossible to do and is prohibited by Google and Apple policies.
Based on our experience, it is better to explain children the purpose of the app. You can tell your child that it is the app for watching parents as well. For example, your child will know where you are, when you get home and he/she will be able to restore order or to say goodbye to friends before you come. Kids will appreciate this.

What to do if you are afraid that a child will delete the app? The app can not be protected from deletion, this restricts the user’s rights. However, in many phones there are options “Parental Control” or “Child Mode.” These options allow you to configure the access to app in the child’s phone. Check if there is such a possibility in your child phone.