Background Measuring competence in children’s ability to make decisions about their care is laden with conceptual problems. Although there are a variety of tools to measure competence, no scales have been identified that measured competence in children.

Aim To develop, test and validate a scale that measured competence in the decision-making process of children aged 8-12 years with long-term conditions, called the Children’s Competence in Decision-Making (CCD-M) Scale.

Method A convergent, sequential, mixed-methods validation design was used. Four stages of scale development were used following recommended procedures. The qualitative arm explored the experiences of being involved in decision-making and these insights were used to develop the scale which was then subject to psychometric testing.

Results Cronbach’s α of the overall scale was 0.86, which shows good internal validity. Additionally, the α analysis when items were deleted showed no considerable variation and had no value below 0.7, showing excellent internal validity throughout. No substantial increase in α could be achieved.

Conclusion The CCD-M Scale offers practitioners the ability to test the competence levels of children to decide the extent to which they may want to be involved in the decision-making process. This, in turn, may help to plan care in a more effective way and may have an effect on adherence levels in self-management of illness.