Physical activity (PA) is considered an important part of the treatment of children with diabetes mellitus type 1 (T1DM). Furthermore, health-related quality of life (HRQoL) affects both the physical and mental health of patients with T1DM. The purpose of the study was to evaluate through a randomized controlled trial the impact of participation in a summer diabetes sports camp on the PA and HRQoL of children and adolescents with T1DM. Eighty-four children and adolescents with T1DM were randomly assigned into an intervention (M = 12.64, SD = 1.82, 30 female) and a control group (M = 12.67, SD = 2.50, 30 female). Intervention group participants attended a ten-day summer diabetes sports camp which included an intensive program of PA (6 h of daily PA), educational and entertaining activities as well as education on the importance of PA in the management of the disease. At baseline and at the end of the study, participants completed measures of physical activity, self-esteem, depression, health status, intention to change behavior, and life satisfaction. Results of the two-way repeated measures analysis showed no statistically significant group differences in PA levels (p < 0.05) and HRQoL parameters (p < 0.05 for all parameters). In conclusion, the results did not support the effectiveness of a 10-day diabetes sports camp on PA levels and HRQoL for children with T1DM. Longer interventions may be more effective in exerting positive influence on trait parameters of children with T1DM’s quality of life. Participation in such programs on multiple occasions should be evaluated in the future.