This study investigates the morphological phenomenon known as plurals-in-compounds effect by analyzing responses given by 25 Persian-speaking children. The children, ranging in age from 3.5 to 13, were divided into six groups and asked questions like what do you call someone who verb-s [regular/irregular] plurals? that required them to form synthetic compounds such as car-stealer. Results indicated that when the nouns in the questions addressed to the children were regular plural, the non–heads in the compounds were predominately singular. Moreover, the children’s responses to questions containing irregular nouns showed that they preferred non–compound Agent (singular N) over compounds containing either singular or irregular plural nouns. The findings are in line with the specifications of Kiparsky's (1982) level-ordering model which bans plural morphology inside compounds. It was also revealed that the children’s compound construction relied on both structural and semantic constraints.