Background: The aim of this study was to investigate the influence of transcranial direct current stimulation (tDCS) on the biceps brachii and flexor carpi radialis stiffness in children with cerebral palsy (CP). The authors also aimed to verify the relationship between spasticity and muscle stiffness. Methods: Twelve children with CP (mean ± SD; age, 8 ± 1.3 years; height, 118.7 ± 4.1 cm; weight, 23.0 ± 2.2 kg) were involved in the study. Muscle stiffness was estimated using a MyotonPRO device in a MultiScan pattern of five measurements. Simultaneously, the tDCS stimulation was performed. Spasticity was assessed by a neurologist using the Ashworth Scale. Results: Stiffness of the flexor carpi radialis muscle decreased significantly after tDCS therapy (p = 0.04). There was no significant change in stiffness of the biceps brachii. For all participants, the Spearman rank correlation showed statistically significant and positive relationships between muscle stiffness and the Ashworth Scale (p = 0.04). Conclusions: Transcranial direct current stimulation has a decreasing effect on stiffness and spasticity of the flexor carpi radialis in children with CP. The MyotonPRO device provides objective data and correlates with spasticity measurements.