It is often a challenge for a child to communicate their pain, and their possibilities to do so should be strengthened in healthcare settings. Digital self-assessment provides a potential solution for person-centered care in pain management and promotes child participation when a child is ill. A child's perception of pain assessment differs when it is assessed using digital or analog formats. As we move into the digital era, there is an urgent need to validate digital pain assessment tools, including the newly developed electronic Faces Thermometer Scale (eFTS). This study protocol describes three studies with the overall aim to evaluate psychometric properties of the eFTS for assessing pain in children 8?17?years of age. A multi-site project design combining quantitative and qualitative methods will be used for three observational studies. Study 1: 100 Swedish-speaking children will report the level of anticipated pain from vignettes describing painful situations in four levels of pain and a think-aloud method will be used for data collection. Data will be analyzed with phenomenography as well as descriptive and comparative statistics. Study 2: 600 children aged 8?17?years at pediatric and dental settings in Sweden, Denmark, Iceland, and USA will be included. Children will assess their pain intensity due to medical or dental procedures, surgery, or acute pain using three different pain Scales for each time point; the eFTS, the Faces Pain Scale Revised, and the Coloured Analogue Scale. Descriptive and comparative statistics will be used, with subanalysis taking cultural context into consideration. Study 3: A subgroup of 20 children out of these 600 children will be purposely included in an interview to describe experiences of grading their own pain using the eFTS. Qualitative data will be analyzed with content analysis. Our pilot studies showed high level of adherence to the study procedure and rendered only a small revision of background questionnaires. Preliminary analysis indicated that the instruments are adequate to be used by children and that the analysis plan is feasible. A digital pain assessment tool contributes to an increase in pain assessment in pediatric care. The Medical Research Council framework for complex interventions in healthcare supports a thorough development of a new scale. By evaluating psychometric properties in several settings by both qualitative and quantitative methods, the eFTS will become a well-validated tool to strengthen the child's voice within healthcare.