There's a real problem when teenagers have purchasing power (such as credit cards) but don't have the legal capacity to enter into agreements on their own. The problem of underage purchases is particularly prevalent for online retailers for alcohol and tobacco and they face the same frustrations. While technology is being developed to require more levels of verification for those kinds of businesses (including asking for driver's license information), my understanding is that it's not widely available yet.

However, you may be able to learn from the ultimate goal of the new software: Delaying the transaction to provide the merchant ample time to verify that the transaction is legitimate and made by the real, adult purchaser. Among the steps you may be able to program in connection with your shopping card are: (1) have a verification page that requires purchasers to confirm that they are over the age of 18 and have the authority to make the purchase before you accept the credit card information; (2) seeing if your credit card software can require a telephone number as further verification; (3) have an "opt-in" link that sends a notice to the email address provided once again confirming that the purchaser made the purchase and (once again) verifying the age and authority to use the credit card; or (4) making a confirming phone call to the telephone number associated with the card before delivering the services. Check with an attorney in your area; the falsification of the verification could be enough to warrant criminal or civil penalties against the children making the purchases.