Background: Although early attention to children’s and adolescents’ health education is important, research assessing the health knowledge of children under 13 is limited, and a validated assessment tool has not yet been developed; this forms the rationale for the present study. Aim: Assessing the Reliability of the Mongolian Translation of a Questionnaire for Evaluating Children’s Fundamental Health Concepts. Materials and Methods: This cross-sectional study was conducted in May–June 2025 among children aged 8–11 years attending public general education schools in the Khan-Uul, Bayanzurkh, and Sukhbaatar districts of Ulaanbaatar. Of a total population of 459 eligible students, 352 participated in the study. Data were collected using a structured questionnaire consisting of 12 sociodemographic items and 15 items from the Health Literacy Assessment Tool for Children (HLSChild- Q15), resulting in a total of 27 items. Statistical analyses were performed using SPSS version 21.0, with p-values less than 0.05 considered statistically significant. Results: A total of 352 students participated in the study. By gender, the majority were girls, accounting for 57.1% (n=201), while boys comprised 42.3% (n=149). Regarding grade level, 24.7% (n=87) were in third grade, 60.5% (n=213) in fourth grade, 11.4% (n=40) in fifth grade, and 3.1% (n=11) in sixth grade. By age, 9.1% (n=32) were 8 years old, 36.4% (n=128) were 9 years old, 43.2% (n=152) were 10 years old, and 11.4% (n=40) were 11 years old. Internal consistency was acceptable, with Cronbach’s α=0.82. Exploratory factor analysis revealed three factors, explaining 56.3% of the total variance. The sampling adequacy was acceptable (KMO=0.596), and Bartlett’s test of sphericity was significant, χ²(105)=2615.78, p<0.001, indicating suitability for factor analysis. Conclusion This study demonstrated that the Mongolian version of the HLS-Child-Q15 questionnaire meets the requirements of reliability and construct validity, indicating that it is an appropriate instrument for assessing the foundational concept of health literacy among children in Mongolia.