Herbal medicine for a child who doesn't eat well

My child is of average height and weight... but the amount and speed of eating are very slow, and I think they are picky eaters too. So, from others' perspective, it seems like a child who doesn't eat well? That's the feeling I get.

So, I think I've really tried all the medications, like the height-increasing medicine.

Actually, there is medicine that helps improve appetite. I want to give it to them, but the adults say that if someone doesn't eat well, there is a herbal medicine that makes them eat better. They tell me to give that. Is there really such a herbal medicine?

There are also sayings like "I didn't take herbal medicine properly and my constitution changed, so I gained weight."

Can it really happen that way?

I'm curious.