Hi Carol,
It sounds like you are looking for something similar to a kiosk application. I saw that MobiLock has been mentioned, but unfortunately, you must pay for the application after 30 days. My company has been using
Clyd Kiosk Standalone Lockdown to lock down our mHealth platform in Liberia. The application is free, does not require internet connection, and should be available in your country. It essentially creates a new home screen where you can add whatever apps that you'd like. It also allows you to add hidden system features in case you want to allow users to use CommCare's other features such as the camera, GPS, wifi-direct, etc. While the App doesn't have any advertisements, it does have an annoying bar at the bottom of the screen featuring Clyd's other services.
Hope this helps,
Nick