This study evaluates the Children's Worlds Psychological Well-Being Scale (CW-PSWBS) within a diverse international cohort of children aged 10 and 12, utilising Classical Test Theory (CTT) and Item Response Theory (IRT) methodologies. Through a detailed psychometric analysis, this research assesses the CW-PSWBS's structural integrity, focusing on its unidimensionality and measurement invariance across gender and age groups. The study also analyses the scale's item discrimination and difficulty levels using IRT. Participants: The sample consisted of children from 30 countries, aged 10 and 12 years, offering a broad perspective on psychological well-being across different cultures and developmental stages. The CW-PSWBS demonstrates a unidimensional structure, ensuring consistent measurement across genders and age groups. The scale's items exhibit strong discrimination and appropriate difficulty, highlighting its effectiveness in capturing the latent trait of psychological well-being, particularly at low and average score ranges. Validated by a comprehensive analysis grounded in both CTT and IRT, the CW-PSWBS emerges as a reliable tool for assessing the psychological well-being of children aged 10 and 12 across a wide array of cultural contexts. The study affirms the scale's robustness and cross-cultural validity, making a significant contribution to the field of child psychology.