Background and aims: Control of type 1 diabetes (T1D) is a daily challenge in children because of highly variable insulin needs. We assessed the safety and efficacy of automated insulin delivery (AID) in free-life in pre-pubertal T1D children while used 24/7 vs. evening & night (EN). Materials and methods: One hundred and twenty-two pre-pubertal T1D children treated by insulin pumps (CSII) were enrolled in a multicenter prospective open label randomized control trial comparing glucose control with 24/7 or EN use of the hybrid Tandem t:slim X2 with Control-IQ AID system for 18 weeks. After a 3-week run-in phase for Tandem X2 pump and Dexcom G6 CGM training, AID was activated 24/7 or EN according to randomization. Primary outcome is %time spent in 70-180 mg/dl target range (%TIR); secondary outcomes include average CGM, %time below and above target range (%TBR and %TAR, respectively), and the same metrics during day-time and overnight. Results: Patient characteristics at inclusion were: 49F/73M, age: 8.6±1.6, T1D duration: 5.2±2.3 years, CSII use: 4.6±2.5 years, HbA1c: 7.7±0.7% (61±5.3 mmol/mol). Except for 3 early drop-outs, 119 completed the trial. AID was effective for 93.6 and 50.9% of time on 24/7 and EN modes, respectively. The increase of %TIR was significantly larger on 24/7 vs. EN mode: 52.9±9.5 to 67.5±5.6 (+14.5%, CI: 12.4-16.7%) vs. 55.2±10.8 to 64.7±7.0 (+9.5%, CI: 7.4-11.6%), p<0.001. While mean %TBR was similarly reduced: 4.4 to 2.6 (24/7) and 4.6 to 2.7 (EN), mean %TAR decreased significantly more on 24/7 vs. EN mode: 42.9 to 29.8 (-12.2%, 95CI: 9.7-14.6%) vs. 39.7 to 32.3 (-7.4%, 95CI: 5.0-9.9%), p=0.008. Increased %TIR was superior on 24/7 vs. EN mode during daytime: 54.3±10.1 to 62.6±6.2 vs. 55.7±11 to 58.4±7.6, p=0.001, and reached 77.2% on average overnight on both modes. CGM levels (mg/dl) more significantly decreased on 24/7 vs. EN mode: 171.9±20.3 to 158.1±10.6 vs. 168.9±20.8 to 162.6±12.1, p=0.02, while % of patients with HbA1c level<7% (53 mmol/mol) moved from 14 to 36 (24/7) vs. 13 to 22 (EN), from baseline to week 18. SD of CGM levels (mg/dl) was also reduced significantly more on 24/7 vs. EN mode: 69.6±9.8 to 61.5±8.0 vs. 69.8±11.3 to 66.7±10.0, p=0.0001. %TIR was increased through the whole range of baseline HbA1c and %TIR levels and always more with 24/7 use. No ketoacidosis or severe hypoglycemia occurred during the study. Conclusion: Our study demonstrates the safety and efficacy on glucose control of Tandem Control-IQ AID system used in free-life in pre-pubertal T1D children for both 24/7 and EN use. 24/7 use shows better performance than EN use, with 14.5% more time in range, or more than 3.6h/day, with no safety issue. The post-study extension for 18 weeks will assess the sustainability of 24/7 use of AID in the full study cohort.