A normal gait is called a heel-toe gait and that means that as one leg swings forward, it lands with the heel on the ground first. Then the body proceeds over that stance phase limb and finally the foot rises up on tip-toe right before it swings forward again. When the leg is on tip-toe, the other leg is on heel strike so in normal gait there is never a time when a child should be on both tip toes.
One of the more common reasons is that one leg is quite short and if the leg is more than about 3 cm short, a child will often compensate by tip toeing so that the leg reaches the ground. A child that is spastic in one leg or one side of the body may tend to tip toe on that side because of the overactive Gastrocnemeus (calf muscle). A patient who has severe Achilles Tendonitis (pain in the back of the calf muscle) or severe Calcaneal Apophysitis (heel pain) might tip toe to take some of the tension off their Achilles tendon. A rarer cause of a child tip toeing on one side only could be a deep muscular calf Hemangioma. This is a vascular neoplasm which causes swelling of the calf muscle.
The most common cause is idiopathic toe walking (no known cause), also called habitual toe walking. Walking on tip toes is quite common between 10 and 18 months when children are learning to walk. In some children it simply becomes a habit, when asked to walk normally they put their heel down on the ground before their toes. It’s just that when they’re not concentrating they seem to revert to walking on their toes.