Childhood overweight/obesity poses a significant public health burden, closely linked to time allocation across various movement behaviors. We aimed to clarify the compositional associations between 24-h time allocation to sleep, sedentary behavior (SB), light physical activity (LPA), and moderate-to-vigorous physical activity (MVPA) and overweight/obesity risk among children aged 2-6 years. This cross-sectional study enrolled 5372 children aged 2-6 years from Beijing. Isotemporal substitution modeling and weighted quantile sum (WQS) regression were adopted. Among all children (mean age 4.52 years; 49.9 % girls), 26.13 % were overweight or obese. Each additional 5 min of daily SB was associated with a higher odds of overweight/obesity (odds ratio [OR] = 1.10, 95 % confidence interval [CI]: 1.02-1.19, p = 0.02), while each 5-min increment in sleep was linked to reduced odds (OR = 0.91, 95 % CI: 0.84-0.98, p = 0.02). Isotemporal substitution analyses indicated that replacing 5 min of SB with sleep (OR = 0.81, 95 % CI: 0.67-0.97, p = 0.02), LPA (OR = 0.84, 95 % CI: 0.72-0.98, p = 0.03), or MVPA (OR = 0.87, 95 % CI: 0.76-1.01, p = 0.06) was associated with lower overweight/obesity risk. Replacing SB with sleep or physical activities reduced the risk. Further WQS analyses revealed that sleep exerted the strongest weight in the behavioral mixture influencing childhood overweight/obesity. This study provides evidence that theoretical reallocation of sedentary time to sleep or physical activities was associated with a significantly lower risk of overweight/obesity in children aged 2-6 years. Importantly, sleep appears to be the most potent component in the behavioral mixture, reinforcing the importance of holistic, multi-behavioral approaches in early childhood obesity prevention strategies.