Or have a well behaved child. My wife took our son to see her family. He didn't even whine the whole trip and it had a layover too. He was 6 months old at the time.
This.
Or the parents who spend the entire trip going "shhhHHHH" to their noisy kid, who eventually will simply ignore the sound entirely. The kid has learned well: Failure to follow commands results in no consequences whatsoever, likely from a parent who won't instill discipline, for fear of scarring the kids fragile wittle ego.
My annoyance at the child's noise is greatly overshadowed by feelings of (sympathy? embarrassment?) for the parents.
I will admit to some of this, for parents who look like they're at least acknowledging the disruption, and trying to quiet the infant properly. One mother I saw would yawn at her baby when the pressure change really kicked in. The baby started to yawn as well, and he quieted down pretty soon after that.
Then there are the parents who will just go on business as usual, while their toddler-age or elementary-school-age offspring continues to make loud noise whenever possible.