Blood sampling is a common and necessary procedure in the treatment and diagnosis of a variety of diseases. However, it often results in painful and stressful experiences for children. Designed together with domain experts, Pufferfish is a breath-controlled biofeedback game technology with bespoke airflow sensor that aims to calm children during blood sampling procedures. An initial randomized controlled trial was conducted in which 20 children aged 6-11 were assigned to one of two conditions involving either passive distraction (watching a video) or active distraction using the Pufferfish prototype. Medical staff rated Pufferfish significantly more useful in facilitating the blood sampling procedure compared to passive distraction. Qualitative feedback from patients, parents, and medical staff identified aspects that impact the acceptance of breath-based active distraction. Our study highlights the potential of non-pharmacological assistive technology tools to reduce fear and pain for children undergoing painful or stressful medical treatment.