In Chapter 21, Holden happens upon his little sister, Phoebe, sleeping peacefully in his brother D.B.'s room. After watching her, he decides that children always look very peaceful when they sleep, unlike adults. It makes sense that Holden would say this. He is a character who is quite thoughtful and critical about everything. He tends to see things in a more cynical light than other people his age, and it shows in his observations of others. In life, he experiences a feeling of discontent with everything; this contributes to his views on the adult pursuasion and his judgemental attitude toward virtually everyone else that he runs into.
Adults, he thinks, are very "phony" in general. In fact, he sees a lot of what happens in the world as their fault. We know that he has experienced some trauma in his past which makes him a troubled person and an unreliable narrator. It also gives him a unique perspective on certain situations. Where someone else might look upon Phoebe sleeping in a positive light, he marvels at how different she looks than an adult would. This negative spin that he puts on everything that he witnesses speaks to his internal anger and critical nature.