Well, first, I would try to pin down when the "one thought that completely broke" you occurred, along with what might have been occurring in your life, and try to identify any correlations between the thought(s) and what's happened around you.
Second, I would like to point out, as had to be pointed out to me, that if these thoughts are hurting you so badly, and causing you such distress, then I would think that means that you still know and feel it is not OK to harm children.
I would like to ask though, what you mean when you say "even when I see the topic or hear it I feel nothing"? Because since you feel as though you can't even be around children anymore, and that this is extremely upsetting to you, I'm inclined to believe that you still very much feel things from it, and it is rather more likely that processing the thoughts/imagery is extremely difficult and stressful. All of that is natural.
There have been an unfortunate amount of intrusive thoughts and unwanted mental imagery regarding children, that tormented me for so long, too.
After therapy, and proper medication, this has gotten infinitely better, and I am able to look back now, recognizing that those thoughts were so intense because of how very much I care and wish to prevent children from harm.
Remember, your thoughts, and the images, do not define you, or who you are. Try to understand that the terrible way those make you feel, is a better representation of who you are, as well as what you do and do not support.
Cognitive Dissonance is brutal like that though, and often ends up making things burn in our minds, as we try to comprehend how we are even capable of such horrible thoughts.
OCD is a beast that will always do its best to convince you that you are a monster, and it will do a good job convincing you of the worst- but OCD is not what tells you who you are and what you are about.
I cannot recommend enough that you find a therapist that fits you, and be completely candid with them about these tortuous thoughts and feelings.
You're not alone, and you can do this.
If someone like me was able to pull myself out of the dark waters, with the help of therapy and eventually medication, then I assure you that you can do it too.
