The initial phase of linguistic production by children is characterized by rote-learned, lexically restricted forms and constructions. Only during later phases of language acquisition do they develop flexibility across a paradigm and mix lexical and grammatical material more freely. In the development of verb morphology, a correlation between the use of tense and aspect has been observed in many languages. It has been suggested that this leads to an intermediary state of paradigm categorization based on temporal categories. So far the flexibility of individual verbs occurring in different tense-aspect combinations has not been examined in detail. Here we evaluate the flexibility of verb use in a large longitudinal corpus of 4 Russian children. We compute the Shannon entropy of verb stems distributed over individual grammatical forms. Results show that children do not pass through a stage of paradigm categorization based on aspecto-temporal categories. After a brief item-specific phase of rote learned forms, they quickly become flexible users of verbs in both aspects.