I did exactly the same for my 12 yo son for a trip to LIS as the OP did. I didn't bother amending the ticket, and no questions where asked at check-in.

I've also made the same mistake on our forthcoming paid fare from ATH, and given my LIS experience, and since we're well outside the 24 hour window, I will not try to amend.

Adult and child fare price/taxes are the same for all European destinations, so as long as name matches the passport, salutation doesn't look like it matters. World-wide fares are a different matter as there's a 25% discount for a child fare.