That freedom of action is generally predicated on the idea of competent individuals and businesses interacting voluntarily. When dealing with children that rationale goes out the window, because children are not competent to voluntarily participate in the economy. There is very little that is more in the province of the state to micromanage, even in a libertarian utopia, than interactions between businesses and people who are legally incompetent, like children.

If your counter-argument is predicated on businesses freely exchanging with parents to convince them what products to feed their children, I fully agree that they should be free to do that, but that's distinct from advertising to children directly.