Final answer:
Children in middle childhood can usually balance on either leg for 30 seconds or more by the ages of five to six, as their gross motor skills, balance, and coordination improve significantly during this developmental stage.
Step-by-step explanation:
In terms of gross motor skills, a child in middle childhood should be able to stand and balance on either leg for 30 seconds and beyond by age five to six. By this age, children's muscle strength and coordination have improved significantly, enabling them to perform tasks like walking on a balance beam, turning somersaults, and balancing on either foot with good control for about ten seconds. As they continue to develop gross motor skills, their ability to balance for extended periods tends to improve even further, and by the ages of six to ten, movements become more controlled and graceful. Typically, during this period of middle childhood, children learn to ride a bicycle without training wheels, indicating a higher level of balance and coordination.