The study determined the ED90 of intravenous remimazolam for the reduction of preoperative anxiety in children aged 1–6 years undergoing elective surgery under general anesthesia. Children were divided into two age groups: a younger group (YG, <4 years) and an older group (OG, 4–6 years), with a total of 80 patients (40 in each group). A biased coin design (BCD) was used, with a positive response defined as a Parent Separation Anxiety Scale (PSAS) score <3; the initial dose was 0.2 mg/kg and was adjusted by 0.05 mg/kg. The ED90 (90% confidence interval) was 0.20 mg/kg (0.17–0.24) in the YG group and 0.15 mg/kg (0.11–0.17) in the OG group. The difference between groups is statistically significant as the 83% confidence intervals (YG: 0.18–0.24; OG: 0.12–0.17) do not overlap. Older children require lower doses based on weight. The study ran from April to August 2025 and is registered under the identifier ChiCTR2500098747.