We can all imagine the situation, you eventually get onto your flight and find your seat, perhaps take-off has been delayed by fog, but you're finally there.
You settle yourself down after stowing the overheard luggage, and all of a sudden you hear the unmistakeable sound of a crying baby. It's not what anybody wants, but it's likely that the parents are far more aware of it than you are.
It's hard to know whether to offer to help, or to simply try your best to ignore it and complain to yourself.