This study examined the effect of a 10-week activity tracker, companion social website, and text messaging intervention on body mass index (BMI), body fat percentage, physical fitness, and physical activity (PA) self-efficacy of children. Children (n = 98) were assigned to one of three groups: activity tracker group, text messaging group, and activity tracker + text messaging group. Pre-/posttest fitness, BMI, PA self-efficacy, and body fat percentage were measured. The activity tracker group had a significant increase in fitness, no significant difference in PA self-efficacy, and an increase in body fat percentage and BMI (p < 0.05). The text messaging group had a significant increase in BMI (p < 0.05). The activity tracker + text messaging group had no significant differences in fitness, BMI, PA self-efficacy, or body fat percentage. This study gives no clear support to the hypothesis that use of activity trackers and a text messaging intervention have a positive effect on fitness, PA self-efficacy, or body fat percentage.