Objective: To develop and validate a difficult intravenous access risk assessment and escalation pathway, to increase first time intravenous insertion success in paediatrics. Methods: Mixed methods underpinned by literature and co-production principles. Iterative development of the instrument was informed through semi-structured interviews and stakeholder workshops. The instrument includes a risk assessment, inserter skill self-assessment, and escalation pathways. Reproducibility, reliability, and acceptability were evaluated in a prospective cohort study at a quaternary paediatric hospital in Australia. Results: Interview data (three parents, nine clinicians) uncovered two themes: i) Recognition of children with DIVA and subsequent escalation is ad hoc and problematic; and ii) Resources and training impact inserter confidence and ability. Three workshops were delivered at monthly intervals (February–April 2020) involving 21 stakeholders culminating in the co-production of the “DIVA Key”. The DIVA Key was evaluated between May–December 2020 in 78 children; 156 clinicians. Seventy-eight paired assessments were undertaken with substantial agreement (concordance range = 81.5 to 83.0%) between the assessors. Interrater reliability of the DIVA risk assessment was moderate (kappa = 0.71, 95% CI 0.63–0.80). The DIVA Key predicted multiple insertion attempts for red (high risk) DIVA classification (relative risk ratio 5.7, 95% CI 1.2–27.1; reference low risk). Consumer and clinician satisfaction with DIVA Key was high (median (IQR) = 10 [8–10]; 8 [8–10 respectively). Conclusion: The DIVA Key is a straightforward, reliable instrument with inbuilt escalation pathway to support the identification of children with difficult intravenous access.