<p>The purpose of this study is to objectively quantify physical activity and sedentary behaviour in children and adolescents during hospital admission using device-based monitoring and investigate the association between sedentary behaviour and pain, constipation, surgery status, movement restrictions, 30-day readmission, and length of stay. Children and adolescents aged 2 ≤ 17&#xa0;years admitted between March and September 2025 to three paediatric departments at a Danish hospital were eligible for this prospective observational study. Children and adolescents scheduled for day surgery and wheelchair users were excluded. All participants wore two triaxial accelerometers throughout their hospital stay. In total, 103 hospitalised children and adolescents were included. The participants spent 45&#xa0;min per day (interquartile range 20–125) in upright position (standing and walking time combined) and were sedentary (sitting and lying time combined) for 23.3&#xa0;h per day (21.8–23.7). Adolescents undergoing spine surgery were sedentary for 23.6&#xa0;h per day (23.5–23.7), of which 22.6&#xa0;h (21.8–23.0) were spent lying in bed. Sedentary behaviour was significantly associated with higher age (<i>p</i> &lt; 0.001), opioid consumption beyond standardised analgesia (p = 0.028), and orthopaedic procedures (<i>p</i> &lt; 0.001). No associations between sedentary behaviour and length of stay (<i>p</i> = 0.391), 30-day readmission (<i>p</i> = 0.467), laxative use (<i>p</i> = 0.695), and postoperative movement restrictions (<i>p</i> = 0.637) were found. </p><p><i>Conclusion</i>:&#xa0;Hospitalised children and adolescents in this study exhibited high levels of sedentary behaviour. The findings highlight that mobilisation strategies are needed across all paediatric surgical procedures, but adolescents, patients undergoing orthopaedic or spine procedures, and those prescribed opioids beyond standardised analgesia require particular attention. There is a need for further evidence of the consequences of sedentary behaviour during hospitalisation among children and adolescents.<Table Float="No" ID="Taba"> <tgroup cols="1"> <colspec align="left" colname="c1" colnum="1" /> <tbody> <row> <entry align="left" colname="c1"> <p><b>What is Known:</b></p> <p>•&#xa0;<i>Prolonged sedentary behaviour is associated with adverse health outcomes in healthy children and adolescents and in hospitalised adults.</i></p> <p>• <i>Accelerometery provides objective, validated measurements of physical activity and sedentary behaviour in children and adolescents, with advantages over self-report methods.</i></p> </entry> </row> <row> <entry align="left" colname="c1"> <p><b>What is New:</b></p> <p>• <i>These findings highlight low levels of physical activity (767 steps/day) and high levels of sedentary behaviour (23.3&#xa0;h/day) among hospitalised children and adolescents.</i></p> <p>• <i>These patterns remain consistent across surgical procedures and age categories.</i></p> </entry> </row> </tbody> </tgroup> </Table></p>