Final answer:
Two key phases of syntactic development in children include the one-word or holophrastic phase and the two-word phase, representing the beginnings of language construction that enables them to communicate increasingly complex ideas.
Step-by-step explanation:
The process of language acquisition in children involves several stages. Two phases of syntactic development observable in children include the one-word or holophrastic phase and the two-word phase. During the one-word stage, typically between the ages of 12 to 18 months, children use single-word utterances where one word may convey complex meaning. For example, a child may say 'cookie' to mean they want a cookie or they have seen a cookie. This phase is followed by the two-word stage, usually occurring around the age of two, where toddlers begin to combine two words to express more complex ideas, such as 'more cookie' or 'mommy go'.
These stages represent a critical period of language development as children progress from using single words to constructing simple sentences, a stepping stone towards the rich and complex ability to communicate that continues to develop throughout childhood.