@2unfit – welcome to parenting teenagers ! Different set of challenges from when they were little …
First PAYG for teenagers in my view is NOT the way to go, would you want your daughter to have no credit to phone you if she was in trouble, needed collecting late at night ?
You should call the phone company and negotiate a discount, plead lack of understanding, lack of disclosure. You might like to offer them in return to upgrade the contract to unlimited texts.
Blackeberries were popular with kids as if your mates have them messages (BBM) are free
We always got unlimited texts for the kids phones as that’s what they use, these days they use whatsapp (texting which uses your internet allowance which is generally much cheaper/free) or similar