Complex systems of inflectional morphology provide a useful testing ground for input-based language acquisition theories. Two analyses were performed on a high-density (12%) naturalistic sample of two Polish-English children's (2;0 and 3;11) and their parents' use of Polish noun inflection: first, each child's use of inflectional affixes and their lexical restrictedness was compared with their father's equalised sample. Second, the children's spontaneous case-marking errors were analysed in context and measured against type and token frequencies in both parents' data and the child-directed speech (CDS) corpus. Findings in both analyses accord with constructivist theory: near adult-like knowledge of Polish inflections hiding a range of use that is more lexically restricted than in their caregivers' speech; low error rates hiding much higher 'pockets of ignorance' for specific inflectional contexts; and patterns of error that correspond closely to token/type frequencies in the CDS, though with the older sibling making some errors that were not frequency-based. Potential effects of syncretism, case ambiguity and semantics are also discussed.