Xiaomi ships Android with an interface of its own, and it interferes with running apps more than other devices do. Parental control software that gets shut down by battery saving can no longer count time – which is why the ordinary Installation is not enough on a Xiaomi device.
Three settings are added, and this video shows where they are: the app's other permissions, battery saver set to no restrictions, and autostart in the background. All three are set on your child's device.
The Xiaomi specifics are written out in the Xiaomi FAQ, the general route in the Android Installation video and the installation guide. More films are in the video overview.