Well... the real incident...it's in the past. No point in beating yourself up over it. We all do things as kids that we wish we could take back. I did. As for the thoughts now... hey thoughts come into our heads. You haven't acted on them. But I understand how bad it's making you feel. I've had that happen as well. I didn't act on them. They went away. I think I was going through extreme anxiety and stress at the time. Maybe that triggered the thoughts. The thoughts def added to the anxiety.
I think it's really important to bring this up with your therapist. She isn't going to be frightened or judgmental. And she needs to know about the thoughts to be able to help you. You've had your pets for many years obviously and care about them. You aren't going to hurt them. It's more important that the thoughts are there and the effect they are having on you. I really wish I could think of something more comforting to say
I'd trust you with my pets