My understanding is you can swap out Guest #2 on a res. So, swap Dad and Baby.
Are you too late to cancel Plus altogether and get refunded and go Standard? Then, add Plus on board.
At least you know what to do next time. I am thinking Princess - at least on the ship - will not charge Baby for Plus. The PCL call center overseas is going to just read you chapter-and-verse and you have to pay. Maybe your TA needs to try a bit harder. IDK. Seems like they didn't give you the right advice from the get-go.
Hope it works out.