Q. What is it supposed to be on the show? A. A public park. Q. Where is it supposed to be on the show? A.  In or near Miami, Florida. Q. When did we see it on the show? A. We see the park in a flashback in Episode 8, of Season 1.  On the surface, this is simply a day at the park for Laura Mosel and her kids, Dexter & Brian. But the seemingly innocent situation is what ultimately seals Laura's cruel fate. The problem is that Harry Morgan joins her on this trip to the park, and unbeknownst to Laura or Harry, while her kids are playing on the swings, members of the Estrada cartel are watching her.   Harry is recognized as a police detective, and the fact that Laura is fraternizing with him is reason enough for Estrada to order her murder. Q. What is it actually in real life? A. A public park, but far from Florida.

Q. Where can I find it in real life?

A. This scene was filmed at Valley Glen Community Park, at 6150 Atoll Ave, in Van Nuys, CA.

That's in the San Fernando Valley, about two and a half miles east of Deb's high school.

You can see some photos of the park here. 

But don't expect to find this particular playground set at the park.
 
Even though there is an existing playground with swings there (near the northwest corner of the park), the production opted to bring in their own playground equipment (including a larger swing set) to use for the scene. 
 
My guess is that they did it to make the playground equipment look more old-fashioned, since this flashback scene is supposed to take place in the 1970s. 

( I think they may have also added a few palm trees to the park background, to make it look a bit more like Miami. )

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

A.  I didn't. I received a tip that Dexter was filming in this Van Nuys park. 

Here is a StreetView from Google:

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