This study presents the design and development of a multidimensional child poverty index in Colombia that can be used for assessment and policy design. Using a mixed-method design that combines quantitative and qualitative research techniques, we developed a multidimensional measure of child poverty. We conducted 80 focus groups with children and adolescents in four regions of Colombia, as well as 27 interviews with experts and policy makers in order to incorporate the voices and perceptions of children, youth and experts in the index design process. Based on these results as well as empirical evidence on deprivations that are determinant for human development, we constructed a Multidimensional Child and Adolescent Poverty Index (MCAP). We used nationally representative household survey data to estimate the MCAP for 2008, 2010 and 2011 in order to measure changes over time. We also decomposed the MCAP in order to identify critical dimensions for different age groups and estimated the MCAP for different departments and regions in Colombia. We found that overall, 34% of children and adolescents in Colombia are poor. This represents a 10 percentage-point reduction in comparison with 2008, which is important progress. We also found significant heterogeneity by regions: while in Bogotá MCAP is less than 20%, in Chocó and Guajira, more than half of children and adolescents are poor. As per the critical dimensions, we found that for all age groups lack of access to potable water, overcrowding and lack of access to parks or green areas contribute the most to child poverty. In addition, for children under five and adolescents, lack of access to education is a critical area and for children and adolescents, lack of access to recreational or time use services has an important contribution to poverty. We show how the MCAP can be used as a policy design tool for child poverty reduction that is sensitive to the needs of children from different age groups and regions through the design of benefit packages. Both the mixed-method research design and study findings are expected to have a significant impact across sectors including both academic and international and national policy circles.