If you want to remove the app from the child device, please use the built-in uninstall routine. You can access this routine by accessing Child Control app and clicking Uninstall App in the About section (see screenshot below).
This is the only way to remove the device from the web portal and to release any used license for use with other devices or products (e.g. the Windows version of Child Control). If you would like to save your settings before uninstalling, you can save them in the Web Portal under Settings -> Import/Export and import them again on the new device in the same way. Please remember that after uninstalling, the settings of the device in the Web Portal are permanently deleted.
Please make sure that the device has an Internet connection when uninstalling, otherwise the device cannot be removed online in the Web Portal and the license remains in "is used" state. If the device is no longer available (e.g. due to a technical defect or loss) you can delete the data directly in the Web Portal for this device. Select the corresponding device in the upper right corner and then navigate to Settings -> Delete device.