Background: The spleen is the most commonly injured organ in children. There is variation in care which guidelines aim to address. Most guidelines focus on management in a paediatric hospital, yet 2/3rd present in other settings, with varying resource levels. To address this, we developed a new guideline to support splenic injury management across settings with varying resource levels. We plan to optimise the guideline through an implementation trial. Purpose: To co-develop the components of an implementation optimisation trial, which aims to ensure the new guideline can be successfully, sustainably implemented, in any hospital context in which a child with suspected splenic injury presents. Methodology: Participatory research methods were used to design and run a series of four x two-hour interdisciplinary on-line workshops with 130 clinicians from around NSW, and a group of consumers with trauma system experience. The first workshop used problem trees to elucidate issues with existing guidelines and care of injured children, the second focused on solution identification in each hospital context, and the third on developing interventions to support guideline uptake and use. A draft NSW-context-specific guideline was also presented for feedback. In the final workshop, outcome measures were developed in relation to appropriate care, and consumer and clinician satisfaction. Results: A guideline decision-making tool was developed, along with a suite of interventions to support its use in any context, including: elements of preparation; champions and mentors; design and accessibility; education and training; communication; and external supports. Outcomes of all workshops were presented as a program logic model, forming the basis of a design for a paediatric splenic injury guideline implementation and optimisation trial. Conclusion: Participatory research methods have resulted in a draft splenic injury guideline applicable in multiple contexts and a robust optimisation implementation trial design for sustainable introduction across the trauma system.