Want to automatically pause devices

I can set the hours for which my four kids can use the internet, but I want to set a limit, to where it automatically shuts off after a certain period of time. Currently it just sends a warning to me through Xfinity, which is hard to detect, and then I have to manually go in to pause their device. Amazon tablets can set a time limit of use and then they literally shut off when their time limit has been reached. Can Xfinity ? I currently can't find a way, and I don't want my 4 kids to be online all day.