Pain is a common occurrence in children after surgery, but it has been neglected over the years. Management of pain is a fundamental human right for everyone who undergoes any surgical procedure. Pain assessment is the initial step of pain management and involves subjective and objective (behavioural and physiological) measures. This study aimed to describe the challenges of registered nurses in assessing post-operative pain among children (0- 3 years)). Descriptive phenomenology was adopted. Purposive sampling was used to recruit nine registered nurses with experience in nursing children after surgery in Effia Nkwanta Regional Hospital (Ghana). The researchers conducted an in-depth interview which was audio-recorded, transcribed verbatim and qualitatively analysed following Colaizzi’s approach to descriptive phenomenology analysis. The study findings revealed that nurses have experienced several challenges in assessing post-operative pain, including inadequate knowledge, inadequate resources, structural challenges, time constraints, and child-imposed factors. Given these results, nurses should make opportunities to enhance their skills and utilize evidence-based approaches to formally assess paediatric post-operative pain.