A. A school playground.

Q. Where is it supposed to be on the show?

A. In Miami.

Q. When did we see it on the show?

A. In Episode 8 (Season 1), "Shrink Wrap".

Dexter goes to see a shrink, not because he needs one (which he certainly does), but because he suspects (correctly) that the psychiatrist is a serial killer who is doing in his patients via a unique method: giving them depressive drugs then convincing them to commit suicide.

During a session with the shrink, Dexter has a brief flashback to his adolescent past.

After a school bully repeatedly picks on young Dexter, he tells his father that he wants to kill the boy. His father, of course, tells him that's out of the question.

But in this flashback, we see young Dexter walking across the playground with a knife in his hand, heading towards the bully, who is alone out on the playground.

At the last moment, Dexter's father appears and stops him.

Q. What is it actually in real life?

A. A school playground, but not in Miami.

Q. Where can I find it in real life?

A. The school is in what I call "The Dexter Neighborhood", because they have filmed so many scenes there. (Actually it's known as the 'Los Altos' section of Long Beach.)

The school is Prisk Elementary School, located at 2375 Fanwood Ave, in Long Beach, CA.

The school is located is right next to the spot where the "Ice Truck Killer" stops his car to check on his victim (Debra) in his car trunk (in the season finale).

In fact, in that scene, you can read a mural on the nearby freeway retaining wall that says "Prisk". That same mural is just barely visible in the distance in this scene, when young Dexter walks (northwest) across the playground, knife in hand.

The school is also just blocks away (east of) Rita's house, Angel's house, Dexter's childhood home, and the Dade City house Dexter inherits from his biological father.

Here is an aerial photo of the school.  And here is a map link.

Q. How the heck did you figure out where it was?

A. This was simple. They have shot so often in this Los Altos neighborhood that it's the first place I look when I'm hunting for a new "Dexter" location. And I already knew that there were two schools nearby, because I passed them while looking for the car trunk scene. So as soon as I saw a school playground, I immediately checked the Live Local aerial photos, looking for the green field next door, visible in the background of the scene, as well as a matching configuration of buildings, basketball courts and fences.