Although sentence repetition is considered a reliable measure of children's grammatical knowledge, few studies have directly compared children's sentence repetition performance with their understanding of grammatical structures. The current study aimed to compare children's performance on these two assessment measures, using a multiple-choice picture-matching sentence comprehension task and a sentence repetition task. Thirty-three typically developing children completed both assessments, which included relative clauses representing a range of syntactic roles. Results revealed a similar order of difficulty of constructions on both measures but little agreement between them when evaluating individual differences. Interestingly, repetition was the easier of the two measures, with children showing the ability to repeat sentences they did not understand. This discrepancy is primarily attributed to the additional processing load resulting from the design of multiple-choice comprehension tasks, and highlights the fact that these assessments are invoking skills beyond those of linguistic competence.