Warts are common, non-cancerous growths in the top layer of skin caused by a viral infection (HPV). Usually children get them on their hands and feet.  They are contagious and are passed person to person, directly or indirectly.  It can take several months from the time of exposure until a wart shows itself.  Children who bite their nails or pick at hangnails are more susceptible to warts. Warts don't cause illness and can be removed. Common warts are the kind that most kids get.  They can be treated with salicylic gel over the counter  treatments which may take many weeks before the wart is gone. A physician can also treat the wart using a solution that causes the wart to cause a blister to form and the dead skin can be clipped away in about a week.  The doctor can also use cryosurgery, using chemicals to freeze the wart.  A scab forms as the skin heals.  Flat warts are smaller and smoother and grow in large groups of 20-100.  These are usually found on the face and are treated by a doctor with a peeling method.Plantar warts are usually found on the soles of the feet and are flattened because the pressure of walking pushes them back into the skin.  These can be painful and are difficult to treat because most of the wart is under the skin.  They can be treated like common warts, but may need to be surgically removed. 
Patrick S. Pasquariello Jr, MD, Children's Hospital of Philadelphia