Hi Nicola well this is how these companies make money. If they send a text to each customer saying that you have exceeded your set credit limit of 50 quid or 100quid then everyone would stop going over their allowance.which would be saying no to business coming in. Ideally if you are giving a phone to the kids then firstly you have to consider a few things in mind like calling up the network and you can set a sum where the phone is cut off if it exceeds the set limit and other thing is international roaming. But guess thas for future nothing much can be done now.