Children's acquisition of variation in the target language depends on a number of factors not yet well understood. This study probes the acquisition of morphological variation in two unrelated languages, Croatian (Slavic) and Estonian (Finnic), focussing on parallel forms of a lexeme expressing a single grammatical category (a phenomenon known as morphological overabundance). We conducted a cross-linguistic elicitation experiment with 140 monolingual, typically developing children aged 3
0 to 6
11 (80 learning Croatian, 60 Estonian). We elicited genitive plural forms in Croatian and partitive plural in Estonian, with lexemes which either are invariant or allow more than one form. Children in both languages were less accurate with lexemes with parallel forms, indicating that the morphological variation hindered acquisition. Pattern type frequency was found to affect accuracy in both languages. Children's choice between two parallel forms was unaffected by age, but significant language-specific differences emerged.