Objective To understand the sleep quality and influencing factors during hospitalization of critically ill children in ICU, and to provide references for clinical development of sleep management programs. Methods A consistent parallel mixed research design was employed, involving 243 pediatric ICU patients. The Children′s Sleep Habits Questionnaire was used for surveys, and monitoring was conducted for sleep, noise, and light levels. Additionally, purposive sampling was utilized to conduct semi-structured interviews with 13 children. Results The total nighttime sleep duration of ICU children was (6.50±1.23) hours, with a sleep efficiency of (78.75±15.39)%. The duration of nighttime noise >50 dB, frequency of nighttime medical and nursing interventions, total number of catheter placements, ICU length of stay, duration of mechanical ventilation, and duration of sedation and analgesia were identified as the main influencing factors of sleep quality in ICU children (all P<0.05). Qualitative interviews revealed 2 themes and 10 subthemes: sleep disturbances were prevalent among ICU children (difficulty in falling asleep and low sleep efficiency, fragmented sleep, disrupted circadian rhythms, frequent nightmares), and multiple factors jointly affected the sleep quality of children hospitalized in the ICU (adverse stimuli from the unique physical environment, enduring pain from ongoing treatment, distress from physiological discomfort, lack of emotional support and companionship, sudden changes in sleep environment and habits, frequent medical and nursing activities). Conclusion Overall sleep quality is poor during ICU hospitalization in critically ill children, and healthcare professionals should promptly identify and actively intervene to improve their sleep quality and promote their recovery.