We buy very little junk food in our home. There is some but we are not big junk food eaters. However my kid LOVES Oreos. She went to the store with a friend and used her money to buy a package. She seemed to think that meant she could eat them all when she wanted too. I told her she could have all of them but she was still limited to how many per day.
She is 10. She didn't see this as quite fair since she spent her own money.