I have as much right to coerce my child as to coerce any adult. If a child is doing something dangerous, then of course you are justified in intervening, just as you would be for anyone else. This is not coercion, it is concern.

You're right, people don't like being told what to do. So don't do it. Explain the situation in terms they will understand, and give them options. Don't want them eating cheetos? Don't buy cheetos. They'll think twice if they have to spend their own babysitting money on junk food.

And of course you are justified in setting reasonable boundaries in your own house. But ultimately, you can't force anyone to do what you want, and it's unethical to use force to do so.