This one is super useful for fantasy/create-your-own-world, but I can definitely see how it might be difficult in a real world setting.

Maybe instead of cultural setting for the town/country you're in, you can focus on the cultural things your character is going to interact with. Are there family traditions that might come up? If it's a busy city, does your character jump into the nightlife or avoid it, etc?

Since your characters are kids, what are the group dynamics you're dealing with. Is one kid the "leader", how did that hierarchy evolve (I envision the old Saturday morning show "Recess", but I doubt your story is quite that... extreme)? Is there a school? what kind of school is it (affluent, low-performing, established teachers, good/poor after school programs or extra curriculars)?