What counts as a business expense for a kid?

Anything the business used up or needed in order to do the work. The test is whether you would have bought it if the business did not exist.

Clear yes: fuel for the mower, trash bags, cleaning supplies, trimmer line, soap, sponges, the materials that went into something you made, printing for flyers or cards, and the cost of anything you bought to resell.

Clear no: things you wanted anyway. New shoes are not a business expense because you wore them to a job. A phone you already had is not an expense. Snacks are not an expense.

The honest gray area is equipment that gets used both ways. A vacuum used for detailing cars and also for the house is partly one and partly the other, and the sensible answer at this scale is to count the portion the business actually used rather than either extreme.

Two distinctions worth understanding early.

Things used up versus things that last. Bags and fuel are gone after the job. A pressure washer lasts for years. Treating a two hundred dollar machine as a cost against one weekend makes that weekend look terrible, when really the machine spreads across every job it will ever do. The useful question for equipment is how many jobs it takes to pay itself back.

Expenses reduce profit, which is the number that counts. This matters beyond bookkeeping. The federal threshold where a kid has to file a tax return is $400 in net self-employment income, and net means after expenses. A kid who never tracked what they spent may look like they crossed that line when they did not.

Keep the receipts. They are annoying and they are the difference between knowing and guessing.