Children and adolescents with complex chronic conditions, including those with life-threatening or life-limiting conditions, are a heterogeneous population. Many individuals exhibit sleep abnormalities that are measurable by proxy questionnaires. No suitable instrument to assess the wide range of different complex chronic conditions is currently available. The aim of the present study was to develop a screening tool—the Sleep Screening for Children and Adolescents with Complex Chronic Conditions—to effectively obtain sleep behaviour information in this population. Following a mixed-method design, potential items for the Sleep Screening for Children and Adolescents with Complex Chronic Conditions questionnaire were defined through literature research and expert meetings. In a pre-test with N = 60 family and professional caregivers, the items’ relevance and comprehensibility as well as the instrument’s overall design were assessed. For the main test, N = 315 participants were recruited in three tertiary paediatric hospitals. A principal components analysis detected the questionnaire’s scales. Item analysis focused on mean values, range, difficulty and discriminatory power. Convergent validation of the Sleep Screening for Children and Adolescents with Complex Chronic Conditions was assessed via correlations between scale items. Most patients had neurological or neuromuscular diseases. Four scales (“Falling and staying asleep”, “Sleep-associated respiration and arousal”, “Daytime sleepiness” and “Sleep-associated movements”) emerged. The item analysis showed satisfactory discriminative power. In the preliminary validation, all scales correlated positively with a child’s care level and with various sleep circumstances items. Three scales additionally correlated with the number of complex chronic condition diagnoses. This newly developed questionnaire can provide clinicians with first indications of possible sleep problems in a growing paediatric population.