Each of our subscriptions allow you to monitor up to 8 devices. So 4 kids each with 2 device would be covered or 8 kids each with one device would be covered.
You are free to removed a device and a new device at any time.
If your child has more than one device you would create a profile for each device. We would suggest making the last name the name of the device. So if you child has an iPad and an iPhone you would have Tommy iPhone and Tommy iPad for devices.
During the setup process you use the simply select the correct device for that profile if they are backing up to the same iCloud.