Children accumulated more steps on weekdays than at the weekend (F = 18.58, p < 0.001, η = 0.27), while boys presented higher PA than girls (F = 12.42, p = 0.001, η = 0.20), and accomplished the recommended daily PA, in contrast to girls. Mothers and fathers presented similar PA (p = 0.67, η = 0.004), with more steps on weekdays than at the weekend (F = 9.22, p = 0.004, η = 0.16). However, their PA was significantly lower than the recommended. Finally, there were no statistically significant associations between parents' and boys' ambulatory activity either on weekdays (p = 0.938) or at the weekend (p = 0.095). On the contrary, parents' PA explained 47% of girls' PA on weekdays (R = 0.68, F = 22.81, p < 0.001) and 23% at the weekend (R = 0.48, F = 6.40, p = 0.02) Conclusions: Boys appear to be more physically active than girls in Greece, whereas mothers and fathers present similar ambulatory activity. Moreover, parents' PA is related to that of their daughters and not to that of their sons. Taking into account that both girls and parents did not meet PA recommendations, family-based interventions are needed for both children and parents benefit.