If you mean like a lemonade stand, I used to do that you should be fine. If you mean sell it like a little shop, or in school you can't do that. One time my friends and I thought, we were young, that we could sell lemonade at the Saturday farmers market, so we went down with our little table and all of our lemonade. But some man crushed our little hearts and said we needed a permit and all of that stuff. But you should be fine selling something on your front lawn as long as yours cops are nice. And if you want to do one of those food trucks you have to have a permit for each town you are going to sell in.
If the child sells food outside the school premises for instance a lemonade stand outside their home, then they do not require a permit. Either way, a business owner can only get a permit if they are 18 and above.