Parents of a child diagnosed with a rare inherited metabolic disease (IMD) are required to actively participate in their child’s treatment, by managing the risk of metabolic crisis and ensuring that they adhere to a stringent diet. In this Research Article, I discuss the specific roles, tasks, and knowledge that Polish and Swedish parents of children with IMDs have developed. I also pay critical attention to the notions and metaphors that parents, clinicians, and social scientists use to describe these. The prevailing metaphors of professionalisation used to describe parental roles contribute to the acknowledgment of these caregivers’ unique expertise. However, I argue, they also overly stress an individualistic perspective and obscure the relationality of care, collaboratively achieved between parents, patients, relatives, and healthcare providers.