Apply 2-3 times a day for a few weeks until the lesion disappears completely and then some. Keep it covered for the first day of treatment (or longer if you want to) to prevent spread. Your child will need to see his doctor for a stronger medication if the ringworm is on his scalp, continues to spread after treatment at home or if it doesn't get better after 4 weeks of treatment at home.
Legal disclaimer: I am providing this general and basic information as a public service and my response to this question does not constitute a doctor-patient relationship. For any additional information, advice, or specific concerns, please speak with your own physician. The information provided is current as of the date of the answer entry.