Kids don’t need to lift weights to build strong muscles. They should be able to build and maintain muscle growth through active play. This can include structured active time in PE or through organized sports, like dance class or soccer. Kids can also build strong muscles through playtime during recess on the playground, playing tag or on family bike rides. There are three elements to building strong muscles in kids: endurance, strength and flexibility.
Endurance: Participating in aerobic activities helps build muscle endurance. Aerobic activity strengthens the heart (which is also a muscle) and helps move oxygen through your body by breathing harder. Examples of aerobic activities for kids include: playing tag, dancing, running, riding a bike, jumping rope, walking, soccer, basketball and swimming.
Strength: Kids build muscles best when they are doing weight bearing activities and lifting their own bodies, which all build strong muscles and joints. Great examples of activities that build strength include: swinging on monkey bars, doing handstands or cartwheels, climbing, tug of war and pushing a friend on a swing.
Flexibility: Kids need to use their full range of motion to help build and maintain their flexibility throughout their childhood and adolescents. Kids can increase their flexibility by: reaching for a ball, doing a cartwheel or backbends, practicing the splits and doing yoga.
Source: Kidshealth.org