Wow, everyone is looking at this all wrong. You were mis-sold a ticket that was not fit for purpose, by the company itself.
A few questions to their prosecutions department and they'd drop it like a hot potato. Particularly when they realised you'd prepared two affidavits, each made on penalty of perjury. And, were considering taking a legal view as regards a countersuit over their failure of duty of care, failure to honour an offer, mis-selling tickers not fit for purpose ... and the resulting embarrassment, distress, costs, undue threats and months of needless anxiety.
After they dropped it... I'd then demand a refund on the child fares paid, and get that too.