Treat your checking account like a swimming pool. Every month, you add 500 gallons of water from child support. You add another 1000 gallons from your job. Maybe you have a birthday and get another 50 gallons for gifts.
Now, you take 300 gallons out to pay your rent. You take 200 gallons out for food. Another 200 gallons for medical care.
How in the world would you be expected to know that the gallons you took out for food, clothing, etc was the same as the money that went in for child care? Obviously, you can't.