The only legally defensible way of countering such an assault is with a single strike, to remove the threat, or restraining him. Anything more, and you're looking at multiple felonies -- even restraining him during approach could have netted a no-fly felony (Interference with flight crew/flight.) Used to fly all the time, crying babies never bothered me, I carried my headphones, and flew business class/first class.
Toddlers as small as the one in this story cannot safely buckle into their own seat, and it's a lot easier to control them in your lap. Flying can be VERY stressful on a child. Think of the age -- teething, probably back teeth at that point, pressure changes affect the inner ear and cause increased pain in the ear/nasal passage/teething areas.
Baby's adoptive daddy is an MMA fighter, but wasn't on the flight. He was PISSED in his interview, said something like "I wish I could have been there, I feel I abandoned my family, If I had been there, it would have gone a lot differently." He implied the guy would be unable to slap ANYONE again, for quite some time.
Dealing with a drunk on a plane -- not fun. Especially one who's physically larger than you and uses that size difference to control space. The woman was scared shitless, and DID move her kid outta the way, just not enough. My lady used to be a flight attendant, and told some stories about the drunks around when she flew the friendly skies -- basically, without an air marshal on the flight, there's limited recourse until touchdown, especially during final descent.