Good Day Doctor I Have Four Grandchildren Of Boys Ages

Good day Doctor I have four grandchildren of boys ages 9,7,3 & 6 months. The older boys are begining to find themselves sexually of realizing what happens when the wake up in the am. Seeing a pretty girl and having the feeling of rising of their penis. I have tried to explain but it is difficult for me. I have told them that is a normal reaction and as the grow older they will understand a little more. My 7 y/o grandson asked why does it hurt when it is expanded it that true. Sometimes when a boy is circumsized and too much come off there wil be pain.