Final answer:
Orthographic Processing is not one of the six key principles of young children's vocabulary development; instead, this principle pertains to reading skills rather than early vocabulary acquisition.
Step-by-step explanation:
The option that is NOT one of the six key principles of young children's vocabulary development is Orthographic Processing. This is because Orthographic Processing pertains to the recognition of the visual form of words and how they are represented in the writing system, which is more related to reading skills and not directly involved in the early stages of vocabulary development in young children.
Other options such as Semantic Network Activation, Joint Attention, and Fast Mapping are indeed involved in vocabulary development. Semantic Network Activation refers to how activating a concept in memory can lead to easier access to associated concepts. Joint Attention is the shared focus between a child and an adult on an object or event, which is critical for word learning.