Purpose: Traumatic brain injury (TBI) often leads to executive functions deficits, which may be responsible for severe and longstanding disabilities in everyday activities. Sensitivity and ecological validity of neuropsychological tests of executive functions have been questioned. The aims of this study were to pilot an ecological open-ended assessment of executive functions in children, the ‘Children's Cooking Task’ (CCT), specifically to report its reliability, discriminant validity and concurrent validity. Methods: Twenty-five children with mild (n = 10) or moderate-to-severe TBI (n = 15), and 21 matched controls (aged 8 to 20 years) participated in the study. An open-ended cooking task was designed to test multi-tasking abilities. It required the preparation of two simple recipes using specific instructions. Outcome measures included the number of errors and an overall qualitative analysis of the task. Validating measures of executive functions included the Delis Kaplan Executive Function System, the Six-Part Test and two questionnaires completed by the child's primary care-giver: the Behavior Rating Inventory of Executive Function and the Dysexecutive Questionnaire for Children. Results: Internal consistency of the Children's Cooking Task was high (Cronbach's alpha = .86), as was test–retest reliability (ICC = .89). Children with moderate-to-severe TBI, as well as children with mild TBI made significantly more errors in the Children's Cooking Task in comparison to controls (p < .001). The CCT was correlated with several tests and one questionnaire of executive functioning (Trails, verbal fluency, sorting, 20 questions, Dysexecutive Questionnaire). Discussion and Conclusion: The Children's Cooking Task has good interrater and test–retest reliability, as well as good discriminant and concurrent validity.