I never told anyone other than my family and years after the fact. The school knew though... the PE teacher caught him in the act after it had been going on for six months (more than just groping... but not penetrative or actual sex) and only because this boy had asked her to tell me over lunch to go out by the tree in the field (idiot boy but good for me). This school was K-12 with all of the ages lumped together (and now I understand personally why that should never be done co-ed). I was about 8 in the 6th grade and he was around 15 in the 11th grade. I was not in puberty and I honestly had no clue about sex or anything else related to it. Totally innocent. But that is what genuine pedophiles actually prey on, right there. The innocence of their victim(s). Despite this boy was legally a child does not excuse it. In old-world law, he was a man. That's what puberty is biologically (18 is just the legal or lawful age... legal/lawful age is not mutually exclusive to biological adulthood or sexual maturity). I was not a biological adult or sexually mature by any means but he was. And that is how they do it, though... to convert you, because it feels good it has to be good... has to be right. It's not and only a fool could claim otherwise (sickeningly, there are people that do think it perfectly natural and alright... they're wrong, wrong, wrong). And for a very long time I was too confused, ashamed and embarrassed to say anything myself (I did not even know what sexual abuse was; how could I? I didn't even know what sex or anything related to it was other than the really, really loose adaptations of TV but still not ACTUALLY knowing you know?). If that PE teacher hadn't come along to check out what was going on it would have continued. But the school never saw fit to inform my family after that point (probably wanted to avoid the potential lawsuit of gross negligence... the abuses were even happening right in the classroom). In any case, somehow after that I was removed from the school and went to another. I do not know the particulars but whatever the school told them (clearly not the truth), that's when it stopped because I wasn't there anymore.
I don't really remember why I said anything in the first place but when I did, my family did not believe me. For years they did not believe me. But then, all of a sudden, that became the reason why I was cross-dressing and wanting to hide my chest and all of this and that... then later, why I was trans when I could finally reveal that as well. And that continued for a while but eventually they came around. I didn't care if they believed me or not, really. I've learned not to care what other people think. It does not matter.
But I never told anyone "official" like a therapist or other about it because I knew that would just lead to roads I did not want to travel (and based on my family's reaction... I was right). It is, for me, best that it s just left alone. Unless I'm actually reminded of it I don't think about it and it does not bother me. That's the way it works for me and I'm fine with it just where it is. However, later on it was for purely unselfish reasons that I came to deeply regret my silence and because that was the point much later on when it dawned on me that my silence was empowering this person to keep doing it. For all I know, that's exactly what's happening with this person still and if he is then it is partially my fault for not saying something to get him officially 'into trouble' to put a stop to it or at least to that there's a record of it. And that is what really eats me about it, not what happened to me but what is happening to others. That is what I have to live with and now that is what I really am ashamed about. I do not know anything other than his first name and so it is far too late now to do anything. That's my hell over it. What happened to me does not affect me or my life but this knowledge does. And it's that which I feel is the reason why I've been able to selectively block it out unless it's triggered. I don't need a therapist to tell me that, see.
This unfortunate thing is completely unrelated to the fact of my being FTM, though. I'm not lying or trying to be cliché when I say I always knew from the very beginning of self-awareness or self-consciousness (for me being around the age of 4 years old), even though I didn't know its name or its particulars just that it 'was'... that *I* was a boy. This unfortunate happening did not even begin remotely around when I started to rebel (in other words, when I seriously began to assert my true gender and my should-have-been 'sex') against the feminine things, around the 1st and 2nd grade on (age 6-7). So to bring it up again in any capacity just does not serve a purpose (I only do when I know it can help someone else otherwise its of no point for me) and so I didn't and don't/won't. There are some things I feel that are just nobody's business (even a therapists) and this was one of them.
In the end, whatever you decide is what is best. I don't believe this would be grounds for causing issues in your transition but I'm not going to lie and say that it couldn't become an obstacle. It depends on the therapist. The first therapist I'd told at age 14 about being trans (when I finally found out what 'it' had been, its name) I was stared at in an almost horror-stricken, slack-jawed expression in response and it all went nowhere (I dropped out of therapy immediately after that, rightly so). Again, the other one I went to to try and get HRT in 2011 was absolutely useless and wanted to skirt the issue. So therapists are a very mixed bag and one should tread lightly until they know the territory... so to speak. Caution is never a bad starting-point with them.
No matter what, I do hope you can attain some closure. That's the important thing. And how that is achieved is personal and there is not a one-fits-all way to get it. It will happen though. Maybe not for a long time but peace will come.
To expound more on the trans side of things. What really grills me is the misconception that medical transition is what is making you male or female. That is only the half of it. You either already are or you are not (or else you're something else entirely, e.g. "androgynous"). I'm speaking of your very self now. Not your body. The body is just the body, a husk of meat really. It is SELF that is male or female, before anything else (this is actually bonafide in science as well because the brain really does get its bath of one or the other hormone to establish gender before the rest of the body gets its hormone bath). Sex is between the legs but gender is SELF (that very thing that makes you self-conscious or self-aware of YOU, the driver of the body/vessel). That is a very important distinction (which unfortunately the vast majority of others just simply do not grasp, including medical professionals if their prose of writing about it is any indication) and it is the only thing that could possibly explain why very young children would even be aware of a discrepancy in their own gender and sex from the other "normal" children before they even actually know what the differences are physically between the SEXES. See what I mean? There's very little room for gray-area for this reason. Being molested or raped has little and more likely nothing at all to do with it (if that were the case, there would be far, far, far more of us than there are. That theory is not true on even just that merit alone) and any therapist worth their salt will be firmly aware of that. But you have to discern your therapist before you do, if you don't want this to potentially devolve into a delay or outright obstacle (and which I personally feel would be worse for both situations and cause more problems than it would solve but then that is my opinion).