The literature generally assumes that object clitic omission is equally allowed in all child languages. In this paper we challenge this claim by means of an elicitation experiment carried out with children acquiring two closely related languages, Catalan and Spanish. Our results show that while omission is high in young Catalan-speaking children, it is very low in Spanish-speaking children. We argue that this difference can be attributed to a property of their respective grammars (the presence or otherwise of past participle agreement when objects cliticize) under the Unique Checking Constraint of Wexler (1998). In a second experiment, we confirm the robustness of early sensitivity to past participle agreement; through a grammaticality judgment task we find a statistically significant difference between the two languages. We show that the parametric approach postulated can be extended, for mandatory, overt objects, beyond Catalan and Spanish, broadening the empirical scope of the Unique Checking Constraint. (Contains 12 footnotes.)