In this study, we focused on the formulation of relative clauses with preposition and genitive pied-piping. Thirty child (5;9 to 8;4) and 30 adult Polish speakers were given an elicited production task and a grammaticality judgment task. Almost all of the children accepted preposition pied-piping, but only half of them produced it. We suggest that the difficulty for the sentence formulator has to do with properties of the structure. We also consider why the structure seems easier to produce in Polish than in some other languages and suggest that a second type of difficulty, which does not occur in Polish, stems from the use of the complementizer as a relative marker. The children did not correctly produce any relativized genitives and accepted them only around half of the time. We claim that case marking complicates the formulation of the genitive construction and that the marginal possibility of genitive stranding delays the acquisition of genitive pied-piping for reasons relating to economy.