Virgin active Refusal to end Contract.

I signed a contract with Virgin Active for my daughter to join the gym, my daughter was under the age of 8 at the time therefore I signed the contract as the payer. I now received an email stating that I now need to pay almost double what I was paying because she is now 18, so I decided to cancel but unfortunately, they are refusing to cancel the contract saying that my daughter must be the one cancelling or else she is liable for the money. Can some explain why someone who did not sign up for a contract can be held liable and I can't cancel a contract, but I signed it? I spoke to a Yaseen van Niekerk.