In there Terms of Service agreement:
"By placing an order through our Site or using the JustFab VIP Membership Programme, you warrant that:
0.you are legally capable of entering into binding contracts;
0.you are at least 18 years old; and
0.you are the authorised user of the credit or debit card used to access the Services."
"If you are contracting as a consumer, you may cancel a Contract at any time within seven working days, beginning on the day after you received the Products or, in the case of JustFab VIP Membership Programme, beginning on the day we send you a Confirmation Receipt. In this case, you will receive a full refund of the price paid for the Products or the subscription fee in accordance with our Refunds and Exchange Policy below.
To cancel a Contract, you must inform us in writing. You must also return the Products to us immediately, in the same condition in which you received them, and at your own cost and risk. You have a legal obligation to take reasonable care of the Products while they are in your possession. If you fail to comply with this obligation, we may have a right of action against you for compensation"
Sorry for the copy and pasting, but you should be able to cancel her subscription, but I don't think you will be able to claim back the money; however, I would remind them that she wasn't old enough to join/sign a contract and did not have parental permission, and that as a good faith gesture they should return the money, otherwise you will take it further.