Self-regulation and metacognition are important for lifelong functioning and can be strengthened with intervention at a young age. Research proposes that musical play enables self-regulatory development, but lacks rigorous approaches to investigate whether a causal relationship between the two exists. We introduced a musical play intervention in a real-world classroom, and examined its impact on self-regulation and metacognition. We adopted a quasi-experimental, pre-test and post-test control-group design, with 98 children aged 6. The intervention group (N = 45) participated in 13 musical play sessions, while the control group (N = 53) had 13 music lessons following the usual music curriculum. Children's self-regulation and metacognition, including metacognitive knowledge, metacognitive regulation and emotional/motivational regulation, was assessed before and after the intervention through three validated instruments: a) an observational assessment of children's self-regulation and metacognition while completing a task, b) a metacognitive knowledge interview following the task, and c) a teacher-reported assessment of self-regulation and metacognition. The results were equivocal: according to the teacher ratings, the intervention group's self-regulation and metacognition improved significantly more than controls following the intervention, but this result was not corroborated by assessment of children's self-regulation on task. Despite this, from the three areas of self-regulation and metacognition, there was a statistically significant improvement in metacognitive knowledge in the intervention group compared to controls, a result indicated by the teacher-reported assessment and by the metacognitive knowledge interview, which revealed a significant effect on children's metacognitive knowledge of strategies. This study can further the discussions on the use of different methodological approaches when exploring self-regulation, and can inform policy and practice in relation to music and play in schools.