One of the more fascinating aspects of pre-school children's verbal language seems to be their uncanny ability to speak in complete sentences. From the time children put two words together meaning fully, they speak in sentences. By the time they arrive in first grade they use simple, compound, and complex sentences. Children can also supply nouns for subjects, verbs for predicates, nouns for objects, and the like.
Frederick, E. C., & Hackleman, B. (1971). An Investigation of First Grade Children's Use of Language Structure. Reading Horizons: A Journal of Literacy and Language Arts, 12 (1). Retrieved from https://scholarworks.wmich.edu/reading_horizons/vol12/iss1/3