What subject does a kid's business count as?

More than one, and which you use depends on what your state expects and what you need to fill.

The most common placements:

Business or Entrepreneurship, as a standalone elective. Cleanest option if your state allows electives to be named freely, and the most accurate description of what actually happened.

Economics. Many states require economics for high school graduation, and running a business covers supply and demand, pricing, competition, margin, and opportunity cost in a way a textbook does not. Worth checking whether your state has specific content requirements before counting it.

Applied Mathematics or Consumer Math. Pricing, profit and loss, percentages, break-even calculations, and tax. Real arithmetic with consequences attached.

Career and Technical Education, where your state uses that category.

Life Skills or Practical Arts for younger students, where the framing is less formal.

There is also a good case for splitting it. A business genuinely spans several subjects, and a kid who wrote flyers, priced jobs, tracked profit, and talked to customers has done writing, math, and economics. Some families count it as a partial credit across two or three rather than a whole credit in one.

Do not overlook the writing. Customer messages, descriptions of what they do, a flyer, and a reflection on what worked are all real composition, and they are the kind a reluctant writer will actually produce because it has a purpose.

Requirements vary considerably by state, from none at all to quite specific. Check yours before deciding how to label it, because the label matters more in some places than others.