When you stand up, if the entire inner border of foot touches the ground it is called flatfoot. Flatfeet are common in children, almost universal before 3 years of age.
Most flatfeet in children are flexible, i.e the curvature of inner border of feet is re-formed when the child is off the feet or stands on tip of toes. It is considered normal (physiological) and is because of laxity (looseness) of kids' ligaments and excess fat underneath the skin of foot.
If the flatfeet are rigid (inflexible) or painful, you should consult your doctor to rule out serious causes, prominent being abnormal bony connection between feet bones (tarsal coalition) and tight heel cord due to neurological problems (cerebral palsy, hereditary neuropathies). Problems with ligaments, muscles, joints, bones and the nervous system can all contribute to flat feet. Many syndromes have flatfeet as a component.
Besides flatness of feet, parents complain about abnormal way of walking (foot turned out) or abnormal shoe wear. When the child complains of pain in feet, especially dull ache at the end of playing, it is preferable to consult the doctor. Pain in knee / hips might also be due to flatfeet.
Investigations are not required in flexible flatfeet. Relevant investigations will be required when other causes are suspected.
Most flexible flatfeet will correct by around 10 years. No shoe modifications / inserts or special shoes (orthotics) are needed unless the flatfeet are inflexible or painful. Please note that special shoes do not change the shape of bones, but only relieve pain. Surgery might be needed depending on the symptoms and underlying causes, if any that warrants such treatment.