Hi, Richard Encarnacion, Mba, Ph.D.. This sounds interesting, but it's more of an idea than a logline.

Actually, the logline is that simple, think reeses tv show.

For a logline, you want something like: "After ______ (something happens), a _______ (the protagonist with an adjective) must _______ (goal of story) so ________ (stakes)."

You might want to add the antagonist in the logline.

Example (I use this example a lot when explaining loglines):

“Dog Treat Heist” (Animation/Comedy) After a group of dog criminals arrives in a small town, an impulsive dog sheriff must defend a dog treat factory so they won't steal food that's meant for hungry dog families.

I see your point about the "Sandbox" logline being that simple. I think the logline can have a better flow. How about: In the Sandbox, a virtual kids playground, children play with friends and go to school.

You could also add antagonists (bullies, school supply monsters, teachers, etc.) -- if these things are in the show.