Professionals in charge of designing individualized plans for children and adolescents in the child welfare system often lack the necessary information, either because it has not been systematically collected or because there are doubts about the reliability of the data obtained. The lack of consensual and validated instruments that gather the necessary information has led to the development of a rigorous and effective form, based on the Delphi methodology, aimed at obtaining an exhaustive knowledge of the characteristics of children and adolescents under the child welfare system. Once a consensus of different specialists approved the hetero-informed form, it was completed by 41 professionals working in residential care facilities for 307 children and adolescents. It consists of 66 items grouped into six dimensions: general information, school/work situation, child welfare system history, family visitation history, biological family information, and experiences of sexual abuse. During its construction and validation, a panel of experts analyzed its format and content during the different phases. Most of the items showed good performance, and professionals highlighted their ease of use and relevance. The method used ensured the content validity of this form. This instrument has proven to be a useful and effective tool for collecting sociodemographic information on children and adolescents in the child welfare system, which may improve their conditions.