Body dissatisfaction (BD) is an important public health issue as it negatively influences the physical and psychosocial wellbeing of children/early adolescents. Available measures of BD for this population are scarce, have a significant bias, or only evaluate weight-related dissatisfaction. This study, through the exploratory factor analysis (EFA), aims to develop and validate the Italian (Study 1) and Spanish (Study 2) versions of a new tool, the Body Image Bidimensional Assessment (BIBA), which is not subject to sex–age–race biases and is able to identify BD related to weight and height among children/early adolescents. Study 3 regards the confirmatory factor analysis (CFA), testing the measurement of invariance across sex and country. The BIBA has a two-factor structure (i.e., weight and height dissatisfaction) according to studies 1 and 2. McDonald’s ω ranged from 0.73 (weight) and 0.72 (height) with good reliability. CFA confirmed the two-factor model as a good fit for the Italian and Spanish samples. Finally, partial metric and scalar invariance of the BIBA dimensions across sexes and nations emerged. The BIBA has proven to be an easy-to-use tool that identifies two BD dimensions among children/early adolescents who could benefit from prompt educational interventions.