Contact the rail company involved, speak to a CS agent there, ask for a refund of the child's ticket.
Explain the situation as fully and as succinctly as you did here, remembering to put the apostrophe in who's.... and you should come out a winner. No rail company wants bad press at the moment. Be professional and courteous, and see what happens.
If there is no joy at the end of this particular avenue, drop the Metro paper a line, stating which journey and train service you were on.
People do read it