The integration of refugee and migrant children, one of Europe’s most vulnerable populations, is a crucial challenge for EU societies and their present and future social cohesion. Despite extensive knowledge on migrant integration, we still lack specific instruments to measure children's integration and set goal-oriented targets to promote it. Our research responds to this need by presenting and discussing a new child-centered system of indicators with a twofold purpose: (1) to assess and monitor the integration of refugee and migrant children across European countries and (2) to guide policymakers and relevant stakeholders. The indicator system's holistic and transversal nature marks an improvement over previous measurement attempts by considering different dimensions of integration; subjective and objective perspectives, as well as contextual factors; and indicators relevant for and comparable across different European countries. The research uses a co-creative methodology that ensures both scientific rigor and relevance for stakeholders and policymakers. A child-centered perspective, active participation of children throughout the research process, and an iterative co-creation design contribute to the system's added value. First-hand experiences, ongoing dialogues, and an ecological approach involving stakeholders at micro, meso, and macro levels (children, families, school representatives, public officials, and politicians) served to develop and validate the set of indicators. As a result, we are offering (1) 14 indicators that assess the integration outcomes of migrant children across five dimensions: access to rights, language and culture, well-being, social connectedness, and educational achievements, and (2) 16 indicators highlighting key barriers and facilitators influencing integration outcomes (i.e., societal and political aspects, particularly school organization and learning support).