Holden's contempt against society is set upon his view that people, in general, have a phony attitude, are hypocritical to their values, and are willing to hurt others unfairly. This, he arguably thinks, is a result of the loss of innocence that comes as a result of growing up. Hence, he is upset about growing up, refuses to mature, and has a fascination for children. His admiration for them is also a result of the fact that they are still at a stage in their lives where innocence prevails. Hence, whatever unfair calamity happens to a child (like it happened to his little brother Allie, who died) is a direct hit to Holden's esteem for innocence and his pain when it is lost.
Phoebe, Holden's 10 year old sister, is in a way more mature than Holden, understands his behavior, and has served as his supporter and only true listener. He appreciates this greatly. In fact, he cherishes how Phoebe, the epitome of the goodness and grace that comes with innocence has given him the honor of being there for him. It is Phoebe that makes him want to become a "catcher in the rye" for the protection of children who play innocently. He is never going to be critical of her: She is the model individual under his pragmatic view of human nature.
A good comparison for the attitude of Holden towards Phoebe is that of his attitude towards Allie, his dead kid brother. Everything that Allie ever did was the greatest, never phony, and so too is everything that Phoebe does.
Holden, in his own mind, will never think that Phoebe will grow up or that she will mature and lose her own innocence. He tries to make sure she never even sees the dirty words someone scrawled in the museum. As the previous poster says, Phoebe herself represents innocence and so Holden can't allow her to change or mature.