What causes recurring ringworm in children?

Recurrent tinea: The most common cause of recurring ringworm is not using the topical medication long enough (it should be applied at least 3 days after the lesion is no longer visible). Another consideration would be re-exposure from a source such as playmate or pet. When the treatment becomes difficult with topical medications, oral antifungals may be indicated. They are available by prescription only.

Ring Worm: Ringworm is a common fungal skin infection otherwise known as tinea. Ringworm most commonly affects the skin on the body (tinea corporis), the scalp (tinea capitis), the feet (tinea pedis, or athlete's foot), or the groin (tinea cruris, or jock itch). http://www.webmd.com/skin-problems-and-treatments/ss/slideshow-ringworm