This paper reports on an intervention study with young Polish beginners (mean age: 8 years, 3 months) learning English at school. It seeks to identify whether exposure to rhythmic input improves knowledge of word order and function words. The "prosodic bootstrapping hypothesis", relevant in developmental psycholinguistics, provided the theoretical framework for the study. Eighty-seven children were randomly assigned to a treatment group exposed to rhythm-salient input in the form of nursery rhymes, a comparison group exposed to prose input, or a control group with no extra input. Results established that prosody can be an important factor in second language acquisition, as in first language acquisition. Children in the treatment group showed improvement in metalinguistic knowledge of English word order but not of function words. This has implications for teaching methods and classroom materials.