For the instance where you paid the penalty fare (or at least I assume you have paid it) then that's the matter closed. So long as you do pay it, you won't be prosecuted for that instance. For any other instances, it's a crime to travel on a child's fare if you are not entitled to it, but not a crime to buy a child's ticket. If I were running GWR I would certainly investigate the matter, but unless you / your Trainline account holder engages in the process, GWR will normally not have the evidence to prosecute you/the account holder. Moreover any communication I suspect will be by email, that's how most fishing trips seem to go. So to answer the question, it's not essential to get a mail redirect.
It's vital that you are not caught doing this again, but particularly if you have other instances in the last 6 months, this can still be prosecuted. If you therefore get another instance within that 6 months window then it will get very difficult for you.