How does a kid track profit?

Two columns and a date. That is genuinely all it takes at this size, and almost nobody does it.

Money in: the date, who paid, and how much. Money out: the date, what it was for, and how much. Write it the same day, because doing it later never happens and a summer of remembering is not a record.

Profit is the first column minus the second. That number, not the first one, is what the business actually made.

The reason this matters more than it looks: a kid who only tracks what came in believes the whole price is profit. They mowed a lawn for thirty dollars and they think they made thirty dollars. The gas, the bags, the trimmer line, and the wear on the machine are all invisible, so the business looks far more profitable than it is and every decision made from that number is wrong.

A notebook beats an app for most kids, because an app requires opening. A page taped inside a closet door works. So does a spreadsheet for a kid who likes spreadsheets.

Two things worth adding once the basics are running.

The date matters more than it seems, because it shows the shape of a month and makes a slow patch visible before it becomes a surprise.

And note which customer, not just how much. That is how you find out that three people account for most of the money, which changes who gets looked after.

The test that this is working: ask what you made last month and what you spent. If you can answer roughly, you are running a business. If you have to think hard about it, that is the gap.