Children with profound cognitive impairment (PCI) are a heterogenous group who often experience frequent and persistent pain. Those people closest to the child are key to assessing their pain. This mixed method study aimed to explore how parents acquire knowledge and skills in assessing and managing their child’s pain. Eight mothers completed a weekly pain diary and were interviewed at weeks 1 and 8. Qualitative data were analysed using thematic analysis and the quantitative data using descriptive statistics. Mothers talked of learning through a system of trial and error (‘learning to get on with it’); this was accomplished through ‘learning to know without a rule book or guide’; ‘learning to be a convincing advocate’; and ‘learning to endure and to get things right’. Experiential and reflective learning was evident in the way the mothers developed a ‘sense of knowing’ their child’s pain. They drew on embodied knowledge of how their child usually expressed and responded to pain to help make pain-related decisions. Health professionals need to support mothers/parents to develop their knowledge and skills and to gain confidence in pain assessment and they should recognize and act on the mothers’ concerns.