Purpose: We investigated the effect of a content knowledge workshop on a teacher's content development in terms of selected task types, task adaptations, and in turn children's task performance during a six-lesson crawl swimming unit in elementary school. Methods: One physical education teacher taught two 6-lesson units of the front crawl before (comparison) and after (experimental) the workshop to different children (n = 88). Task types, task adaptations, and task performance were collected live by trained observers. Results: The number of tasks taught increased from 48 to 61 after the workshop. Informing tasks decreased 28% (p=.005) and applying tasks increased 11% (p=.039). Task adaptations increased from 123 to 211 (p=.021). A significantly higher proportion of tasks was correctly performed by more children in the experimental group. Discussion/ Conclusion: The content knowledge workshop caused a substantial shift in the teacher's content development, which resulted in more children correctly performing the tasks.