Achilles Tendonitis is an overuse injury of the Achilles tendon. Runners and jumpers are often affected. Pain tends to worsen with time, especially after running or jumping. It is treated with rest, ice, wrapping, elevation of the foot, anti-inflammatory medicines, stretching, and shoe inserts.
Sprained ankles are very common. They happen when the ligaments of the ankle get stretched. Elevation of the foot, ice, non-steroidal anti-inflammatories, and rest help it heal.
Post-viral myositis is muscle inflammation after an infection with a virus. Affected kids will have severe pain in the calf muscles, typically within a couple of days of a resolving viral illness (often influenza, but other viruses too). This is a condition that resolves over about 10 days, but medical providers should help with the evaluation of this to be sure the kidneys are not involved. If the urine is very dark it should be evaluated immediately.
A cause of leg pain that doesn't cause limp is Growing Pains.