Children with spastic cerebral palsy (CP) experience altered muscle tone due to biomechanical changes, traditionally assessed through clinical scales. Shear wave elastography (SWE) offers a non-invasive way to quantify these changes. This study aimed to compare SWE measurements in spastic CP and typically developing (TD) children and investigate influencing factors such as joint position, range of motion, demographics, physical condition and, in CP children, the characteristics of CP. It also examined correlations between SWE measurements and spasticity scales in CP children. SWE measured the elastic modulus (kPa) of biceps brachii (BB), pronator teres (PT), adductor longus (AL), lateral gastrocnemius (LG), and soleus (SOL) muscles at rest and during maximum passive stretching (MPS) in 34 spastic CP children (age:​ 3–17) and 44 TD children (age:​ 3–14). Significant differences (p < 0.05) in SWE were found between CP and TD children. CP children had lower values in upper limb muscles and higher values in lower limb muscles at rest, with the opposite pattern during MPS. The Ashworth and Tardieu scales were associated with the elastic modulus in lower limb muscles (AL, GL, and SOL) at rest in CP children. Differences in elastic modulus at rest and MPS between upper and lower limbs and in spastic CP and TD children showed no consistent links to spasticity scales, reflecting neurological dysregulation, muscle architecture, and joint structure involvement. These variations were linked to neurological dysregulation and muscle architecture, with joint structures also affecting. SWE may offer a more precise assessment of muscle spasticity, minimizing the impact of confounding joint structures.