I had this problem as a child also. From the ages of 5-7. My parents finally took me to the doctor and I was found to have a major kidney infection. I had no other symptoms of this, so it was never suspected. I am now 38 years old and have not had problems for 31 years since I was treated for the kidney infection. Good luck. I know this is a very sensitive issue for a child, especially when they are starting to spend the night with friends.