The game focuses on the kids. and their monsters. So I would start with describing the kid of setting you want. Are the kids in suburbia, rural, or a city environment? Then focus your description on the setting from the view point of an 8 year old (or whatever the age of the kids are you are playing). That can be difficult because kids see the world in a very different way from adults.
Then I'd go through character creation and see what kinds of kids and monsters you get. I'd work with the players to define their parents and family but wouldn't allow the players to pick their own family. Give it some conflict and make some challenges in the family life.
The kids would all know each other through school or living in the same neighborhood, so figure that out. Then describe the school, define some bullies, teachers, creepy janitor, and wild monsters and other kids with monsters. The more prep work and things you define ahead of time makes the game run better imo. Then start the players on a normal day of school and let the shenanigans begin.