Accurate assessment of the talo-crural (ankle) joint angle at physical examination is important for assessing extensibility of m. triceps surae (TS) in children with spastic cerebral paresis (SCP). The main aim of this study was to quantify foot flexibility during standardized measurements of TS muscle-tendon complex extensibility (i.e. based on foot-sole rotation) in SCP children, and typical developed (TD) ones. Additionally, we aim to define a method that minimizes the confounding effects of foot flexibility on estimates of talo-crural joint angles and TS extensibility. Children, aged 6–13 years, with SCP (GMFCS I-III, n = 13) and TD children (n = 14) participated in this study. Externally applied −1 Nm, 0 Nm, 1 Nm and 4 Nm dorsal flexion foot plate moments were imposed. Resulting TS origin-insertion lengths, foot sole (φFoSo) rotations, and changes in talo-crural joint angle (φTaCr) were measured. Foot flexibility was quantified as Δ(φTaCr -φFoSo) between the 0 Nm and 4 Nm dorsal flexion conditions. In both groups, φFoSo rotations of approximately 20° were observed between 0 Nm and 4 Nm dorsal flexion, of which about 6° (≈30%) was related to foot flexibility. Foot flexibility correlated to φFoSo (r = 0.69) but not to φTaCr (r = 0.11). For φFoSo no significant differences were found between groups at 4 Nm. However, for SCP children the mean estimate of φTaCr was 4.3° more towards plantar flexion compared to the TD group (p < 0.05). Normalized TS lengths show a higher coefficient of correlation with φTaCr (r2 = 0.82) than with φFoSo (r2 = 0.60), indicating that TS lengths are better estimated by talo-cural joint angles. In both SCP and TD children aged 6–13 year, estimates of TS length and extensibility based on foot sole assessments are confounded by foot flexibility. Assessments of TS extensibility at physical examination will be more accurate when based on measurements of talo-crural joint angles.