Children with cognitive disabilities are at greater risk of experiencing pain. It has been shown that this paediatric population often receive inadequate pain management. Pain may be very difficult to assess, especially in a defined subgroup with non-communicating intellectual disability or severe cognitive disability. Accordingly, several observational pain assessment tools have been proposed to overcome this issue. Due to the absence of an ideal measurement tool, accurate pain assessment requires, after a case-by-case analysis, selecting the more appropriate tool or a variety of combined instruments. The aim of this work is to provide a comprehensive review of the pain assessment tools commonly used in cognitively impaired children. Critical discussion on features and clinical applicability may suggest how to overcome this difficult challenge. Furthermore, this review will help further research aiming to design new instruments and to improve already-in-use tools.