The answer here depends on your child’s device and whether you have access to it.
If you’re installing it on an iPhone, as long as you have access to your child’s iCloud account it can be remotely uploaded and hidden from view in a protected folder. Android users will need access to the device, but once the app is installed, it can be hidden and will run in the background so your kid will have no idea it is running on the device.
With an Android device, you will need access to the phone to install and set up the app. However, once it's on the device, it will run in the background.