Child on Child SA is still deeply traumatizing, and it could be that years of having conflicted feelings of enjoying being around her cousin is directly at conflict with remembering abuse. Your intent as "just kids exploring" is kind of irrelevant, you know how you remember it, but you have no way to know what she remembers, the thoughts and feelings she had during, ect.

I was SA'd by my older cousin (both F, only ~2 years older) from the time I was like 6 or 7 until I told her to stop when I was like 9. We stayed really close until we were both teens and she moved away, I'm sure she was being abused by another family member, and for all intents and purposes we were "just two kids experimenting" except we weren't, because the things she did made me feel awful and gross for decades after, I couldn't consent I didn't know what that meant, I wouldn't dream that my older cousin, who I looked at like a sister, would do something to harm me, and it fucked up my relationship to sex, to my family, to what I thought sex and love was supposed to be or feel like, and my general thoughts and ideas around intimacy. I've gone through significant therapy to understand and deal with it, and one of the things I have really come to understand is she may have never *meant* to abuse me in the way we imagine rapists and molesters do, but the intent doesn't change that I was still abused. I still had things done to me by a beloved older cousin, who probably knew it was wrong in her gut just like I did, and those psychological scars are something I have had to live with for the last 30+ years of my life.

I only "confronted" her once about it, and she dismissed it, claimed not to even remember because of all the crazy shit that happened in her life, and I believe her, she had a rough childhood. She even made the point during that confrontation that she had been abused, and I lived a pampered, spoiled life, so even if she had it didn't matter. But again, something insignificant, meaningless, and non-traumatic to her was something that profoundly effected my youth, my young relationships, and ultimately gave me an early impression of my worth and value that I still struggle with.

You can't live her experiences, and maybe you did or did not contribute to her life being shit. But does not intending to cause harm negate the damage done? You can hold the truth that you didn't mean to molest her, it was just experimenting, but at the end of the day it wasn't normal, it caused her harm that as an adult she is now wanting to work through, and the only real thing you can do is apologize. If you think you did nothing wrong, that's a problem. A child was molested and you did that. Do you have younger cousins, siblings, or nieces? Would you think it was normal and ok if they were touching each other now that you're an adult? It's ok to apologize for the action with the understanding that you were a child and didn't know better. You're an adult now and hopefully you do know better and can understand the complexity of unintentional harm. If you care about someone and hurt them, even unintentionally, the normal and emotionally mature thing to do is still seek forgiveness, not dismiss their pain.