Hi @LauraB46,
Welcome to the forums and thanks for your recent post. Sorry to hear you're having issues with your children's internet usage at the moment.
As per Howard's advice, there is some varying advice in current threads on dealing with this, and as he has kindly pointed out elsewhere we do have the Connect App, which may be able to put you a little more in control. If your children are using laptops, the Connect App would allow you to pause the WiFi to those particular devices so they will not be able to connect to the internet via the hub. However, if they are using mobile phones, they would still be able to tether their laptops to the mobile to run off the data that way, so it would also be worth checking with the individual mobile providers about capping the data allowance if this is the case for your children.
Hope this helps,