Introduction: Children are at increased risk of medication-associated adverse events, often due to weight-based dosing errors. We aimed to reduce the proportion of medications that were administered where the dosing weight was ≥ 10% different from the recorded weight. Methods:_We adopted in-situ usability testing to iteratively improve design of clinical decision support that would enable accurate dosing weight documentation by prompting clinicians to update weight if recorded weight was > 10% different and it had been at least 7 days since the last dosing weight update. Results: The proportion of medication administrations with difference >10% between their recorded weight and dosing weight decreased from 13.1% (56,256/ 429,006) in the baseline period to 9.5% (35,560 / 372,443) in the intervention period (P < 0.001). Discussion and Conclusion: User-centered design of an interruptive alert improved the accuracy of dosing weights during medication administrations without substantial alert burden. In-situ usability testing is an effective approach to rapidly obtain feedback from frontline users and iterate on the design to effect desired behavior changes