The increases in urbanization, pollution, resource depletion, and climate change underline the need for urban planning policies that incorporate blue–green infrastructure (BGI) and ecosystem services. This paper proposes a framework for assessing BGI’s effect on children’s outdoor activities. This effect, called meaningful usefulness, is a central issue due to the influence of experiences with nature on children’s development and the global trend of concentration of children in urban areas. Based on the concept of affordance, the methodology formalizes meaningful usefulness in terms of an index of usefulness of individual settings (IUIS) and a synthetic index of usefulness of BGI in a specific area (ISGI). These are determined via an audit protocol, Opportunities for Children in Urban Spaces (OCUS), which incorporates a set of indicators measuring micro-scale properties of individual places and contextual macro-scale factors. The methodology is applied to BGI components in Cagliari, Sardinia, Italy, which was selected for its superior density of urban green spaces. The application of the OCUS tool confirms its usefulness for investigating functional affordances incorporated into the trans-scalar structures of BGIs. The analytic protocol further contributes to the implementation of urban planning strategies within the smart city paradigm.