Trust me, when one or two loud babies is ruining other peoples flight, it is even worse for the parent or person traveling with the children. My best advice to other travelers is give the parent a break. I lived 4000 miles from my family for over two years and traveled often with an infant and a 6 year old, as a single parent. Kids can pick up on a parent's vibe, like if other travelers were completely rude and acting like we were an inconvenience to the world because we had paid our ticket price and were trying to get home, it made me even more nervous and anxious. I truly felt sometimes that when this happened my kids moved slower and were not cooperative at all. It could have been my imagination but I don't think so. So the best advice I can give is to be a friendly fellow traveler and even offer to help this poor parent get onto or off of the plane. The nicest thing anyone ever did for me as we were loading or unloading was simply offer to carry my diaper bag off of the plane to where my stroller was located. It just really helped my stress level with not feeling as if people were shooting daggers at you and this good feeling helps us as parents remain more calm with our kids thus they respond better to the different surroundings.
Other things that helped my kids were portable movies, story books, coloring books and non messy snacks, ie fruit snacks, dry cereal, sliced apples. With my infant, always making sure that we were reading her favorite book during take off and landing kept her mind and her ears on the story rather than some of the scary sounds that are associated with this process.
With very young children, whom the air pressure changes hurt their ears, nursing, taking a bottle, sucking a pacifier are all great ways to alleviate some of the pressure.
And last but not least, sorry there are just some kids that are whiny, no matter where they are, so have a cocktail, rent a digiplayer, put in your earphones and enjoy!!! And count your blessings that the flight is only 4 hours and you are not stuck with these children forever.