The socio-physical qualities of built environments are, in several ways, of imperative importance for children growing up. The Child-Friendly Cities initiative by UNICEF, an implementation of the UN Convention on the Rights of the Child, has made local governments strive toward child-friendliness. The participation of children and young people is often the focus of such projects, with a potential for a far broader scope. Besides participation processes, what important socio-physical qualities make environments child-friendly, and how can they be developed? This paper presents a structured literature review of the concept of child-friendly environments, in order to address the full socio-physical spectrum. The results focus on concrete factors that have been filtered through child-friendliness and the associated frameworks, showing an inherent dependence between the social context and the physical environment. The shaping of child-friendliness hinges on the realization of environments that are safe, fair, and with accessible and variable green and open spaces. A multi-stakeholder endeavor including, e.g., planners, designers, and managers requires clearly outlined priorities. This study lays the groundwork for further exploration of how the concept of child-friendly environments can lead to positive changes, also as part of the overall strive toward sustainable development.