During the developmental stages, the toddlers tend to say NO to for asserting their independence. It is a very typical behavior of the toddler. In order to assert their independence, the toddlers say NO even when they want to do something.
Saying NO is a mean of asserting independence, Although the climbing out of crib shows that he or she wants to go out. Through this NO answer and screaming, the nurse could conclude that this is a behavior of asserting autonomy.
So, the conclusion of this behavior could be that child wants to play, but by saying NO, he or she is trying to assert his or her independence.