Two studies investigated the validity of an index of transformational complexity in predicting the difficulty of sentence recall and completion by young children. For seven groups of subjects aged from three to seven years, a scalogram analysis yielded high reproducibility coefficents across eight sentence types on a delayed recall task. Children's abilities to complete semantically appropriate sentences of various types was less predictable due to the extreme difficulty of passives. There was a linear relationship between age and frequencies of correct recall and completion. An examination of errors indicated that children strive to produce meaningful sentences despite limited knowledge of complex logical relationships in some sentence types. These limitations appear to constrain the child's grammatical competence. Hence the predicted order of sentence difficulty need not be interpreted as strong support for the primacy of grammatical factors in determining sentence complexity during the acquisition of language.