INTRODUCTION: Children with spastic cerebral palsy (CP) often exhibit significant changes in their skeletal muscles due to altered neural control and atypical mechanical loading. While muscle morphology, in particular muscle size, has been extensively studied, other important properties, such as passive stiffness and muscle composition, remain poorly characterised. Previous studies have typically examined these features in isolation, leaving the interactions among morphology, composition and passive stiffness largely unexplored. This protocol addresses these gaps by providing a comprehensive, multimodal assessment of gastrocnemius muscle properties within a single cohort, which will be systematically compared with control data from typically developing (TD) children. METHODS AND ANALYSIS: This protocol outlines a study to characterise gastrocnemius muscle properties in children with spastic CP and TD peers across different age groups. Muscle size will be measured using three-dimensional freehand ultrasound, composition using quantitative MRI and passive muscle tissue stiffness using shear wave elastography. Additionally, isometric and functional plantar flexion strength will be evaluated through handheld dynamometry and functional strength assessments, respectively. Differences in muscle characteristics between CP and TD children will be assessed, and the relationships among muscle properties will be examined. Exploratory analyses will also evaluate potential associations with isometric and functional plantar flexor strength. This integrated approach will provide a detailed multimodal characterisation of age-related alterations in the gastrocnemius muscle in children with CP. TRIAL REGISTRATION NUMBER: NCT06991725.