Back pain and postural abnormalities are increasingly prevalent among primary school children and may persist into adulthood. This randomized controlled trial evaluated a 12-week web-based preventive back-care intervention for 141 children aged 6–11 years. The intervention group (n = 71) received exercise and back-oriented education videos, while the control group (n = 70) received only educational videos regarding general health promotion. Outcomes included short-term goals such as postural evaluation, back pain, and postural endurance, as well as prevention-oriented outcomes including trunk endurance, functional mobility, back-related knowledge, and psychological well-being (self-compassion and self-concept), complemented by parent-reported information. Exploratory analyses examined subgroup differences in program response. The intervention significantly increased back-related knowledge (p < .001, partial η² = 0.228) and positive self-compassion (p = .011, partial η² = 0.045), but did not lead to significant group differences in posture, back pain prevalence, trunk muscle endurance, functional mobility, self-concept, negative self-compassion, or daily sitting time. Adherence varied, and postural assessment was limited by the lack of a standard medical examination, which should be considered when interpreting the results. These findings highlight challenges in digital health promotion for young children and suggest that future programs may benefit from increased interactivity, parental involvement, and in-person components to effectively support spinal health in this age group.