Complex sentence: There is a relationship between children's prelinguistic phonetic skills and their lexical progress at age two: failure to develop the required phonetic skills in their prelinguistic period results in children's delay in producing words.
Split into simpler sentences: There is a relationship between children's prelinguistic phonetic skills and their lexical progress at age two. Failure to develop the required phonetic skills in their prelinguistic period results in children's delay in producing words.