So for each age, your child needs the right stuff to make the journey. However, we are planning an auto trip this summer when she will be 7 months. When we are prepared, our child enjoys the trip even more. This includes snack, changes of clothes, a travel play bag filled with favorite things, emergency supplies, snacks, juice and water. Certainly in cars we have tie-ups and children can get restless.
Some children have sensitivity to noise.
Sometimes this doesn't happen. I will then firmly instruct the child to stop. If you are so inclined, take a small sock puppet along or even an extra pair of socks and create a little puppet with your hand. Sometimes just that distraction will make all the difference. It's appropriate to ask the parents to be considerate of your needs on the flight and give them the sock puppet to play with.