Question Author
You are all being misled by the debit card. You also (mostly) misunderstand my question. It is a legal one, not a moral one. HPSauce seems to have this understood though and I have followed up on that. As I now understand it, legally, a contract for non-essential items cannot be enforced on an under-18. Yes, the other side can pursue her (an under-18) for lying that she was over-18 (although given how deeply buried in the Ts and Cs this was, and given the lack of attempt they made to verify this fact, they will have a hard job).
The purpose of the question was not to get enough cause to go to court, it was to understand my legal position whilst negotiating with the train company. HPSauce has helped me well, thank you.