My wife and I are booked on separate flights with different airlines between Portugal and Brazil. She's traveling with our toddler on her lap, but due to a few bad experiences, and her flight being a redeye (and mine a day flight), she's pushing me to take our son instead.

I know I can easily add him to my booking at BA, and pay the 10% fare. My question is: if she turns up at the airport without the lap child, will she face issues (cancellation)?