Here's how I think about this:
For many of us, we have a dissociated version of ourselves from the age of our abuse still inside of us. So that when we encounter a boy the same age we were at the time of our abuse, our abuse-age alter feels connected to the real boy, hence the trigger.
Sometimes, we want to connect with the boy as a same-age peer, even though we're grown men and he's a kid.
Other times, like you point out Si, all we can see is the abuse enacted upon him and feel protective and triggered by that.
The key here, in my journey, is to focus on being co-present with your younger self. When your younger part is triggered by the kid, try to see yourself how the kid sees you: as a grown-up, not as a peer. This will help you recognize and understand that you're a man, and that your abuse is in your past and not in the present.
I'll be just fine and dandy
Lord, it's like a hard candy Christmas
I'm barely getting through tomorrow
But I won't let sorrow get me way down.