You can't control your desires, but you can control you actions. Even fantasizing is an action, a passive act, but an act none the less. You feel guilty because you know it's wrong. If you want to stop feeling guilty don't do it any more. Don't fantasize about children. When it pops into your head, think of something else. Humans have the ability to control what they do by choosing which desires to act on. That is what separates us from animals. Over eaters can decide to eat less and get healthy even though they still get cravings for fatty foods. The over eater can't control the desire to eat tasty foods, but they can control what they choose to put in their mouth. In time desires fade even if they never go away completely. The longer one goes without acting on a desire the easier it is to control it. The hard part is going cold turkey and sticking with it.
Children are innocent, you desire to take away their innocence. Nothing good can come from letting that desire take control of you in any way.
