Lost phone, can't access account

I need to stop my daughter's ID SIM because she has dropped her phone irretrievably in a river!  However, I can’t access her account because she is under 18, and when we took out the SIM only contract for it, my email address was used.  However I already had an account attached to that email address and ID mobile can only register one account per email, which I did not realise when I set up the second SIM.   Her account is basically lost in the ether.  We have bought a SIM from a different company for her new phone and I would now like to cancel the contract and get a PAC code to use with the new SIM. Please advise how we can sort this out. Thanks