This article describes a co-design study to develop a sleep hygiene education tool for children with developmental disabilities and behavioural sleep problems. The tool is underpinned by a programme theory, which explains how sleep hygiene education should work to improve children’s sleep. In three co-design workshops, eight parents and six practitioners debated a preliminary sleep hygiene education tool, using themes developed from an earlier evidence review and exploratory study into parent and practitioner experiences of sleep hygiene education. This participatory research established stakeholder acceptability of the SHE tool and confirmed the often-hidden contextual factors that can help or hinder its success, informing the underpinning programme theory.