Early rehabilitation and mobilisation encompass patient-tailored interventions, delivered within intensive care, but there are few studies in children and young people within paediatric intensive care units. To explore how healthcare professionals currently practise early rehabilitation and mobilisation using qualitative and quantitative approaches; co-design the Paediatric Early Rehabilitation and Mobilisation during InTensive care manual of early rehabilitation and mobilisation interventions, with primary and secondary patient-centred outcomes; explore feasibility and acceptability of implementing the Paediatric Early Rehabilitation and Mobilisation during InTensive care manual within three paediatric intensive care units. Mixed-methods feasibility with five interlinked studies (scoping review, survey, observational study, codesign workshops, feasibility study) in three phases. United Kingdom paediatric intensive care units. Children and young people aged 0-16 years remaining within paediatric intensive care on day 3, their parents/guardians and healthcare professionals. In Phase 3, unit-wide implementation of manualised early rehabilitation and mobilisation. Phase 1 observational study: prevalence of any early rehabilitation and mobilisation on day 3. Phase 3 feasibility study: acceptability of early rehabilitation and mobilisation intervention; adverse events; acceptability of study design; acceptability of outcome measures. Searched Excerpta Medica Database, Cumulative Index to Nursing and Allied Health Literature, MEDLINE, PEDro, Open grey and Cochrane CENTRAL databases. Narrative synthesis. In the scoping review we identified 36 full-text reports evaluating rehabilitation initiated within 7 days of paediatric intensive care unit admission, outlining non-mobility and mobility early rehabilitation and mobilisation interventions from 24 to 72 hours and delivered twice daily. With the survey, 124/191 (65%) responded from 26/29 (90%) United Kingdom paediatric intensive care units; the majority considered early rehabilitation and mobilisation a priority. The observational study followed 169 patients from 15 units; prevalence of any early rehabilitation and mobilisation on day 3 was 95.3%. We then developed a manualised early rehabilitation and mobilisation intervention informed by current evidence, experience and theory. All three sites implemented the Paediatric Early Rehabilitation and Mobilisation during InTensive care manual successfully, recruited to target (30 patients recruited) and followed up the patients until day 30 or discharge; 21/30 parents consented to complete additional outcome measures. The findings represent the views of National Health Service staff but may not be generalisable. We were unable to conduct workshops and interviews with children, young people and parents to support the Paediatric Early Rehabilitation and Mobilisation during InTensive care manual development due to pandemic restrictions. A randomised controlled trial is recommended to assess the effectiveness of the manualised early rehabilitation and mobilisation intervention. A definitive cluster randomised trial of early rehabilitation and mobilisation in paediatric intensive care requires selection of outcome measure and health economic evaluation. The study is registered as PROSPERO CRD42019151050. The Phase 1 observational study is registered Clinicaltrials.gov NCT04110938 (Phase 1) (registered 1 October 2019) and the Phase 3 feasibility study is registered NCT04909762 (Phase 3) (registered 2 June 2021). This award was funded by the National Institute for Health and Care Research (NIHR) Health Technology Assessment programme (NIHR award ref: 17/21/06) and is published in full in ; Vol. 27, No. 27. See the NIHR Funding and Awards website for further award information.