Awhile ago I ran into a blog of a public school high school teacher. He was mocking homeschooling and stating that a parent couldn’t possibly know enough to teach their child every subject.
That is true.
Parents don’t know all there is to know about every subject in school, but they can find a curriculum or a teacher that will teach it.
It is simple. If your child is thirsty for water you give it to him. When your water runs out you go find water elsewhere.
So if you child is into medieval history and reads just about everything you can find on it, you can search for something more.
With the age of the internet there are so many options available to parents. Many college classes are offered online that a medieval history buff would love to sink his teeth into. These courses are typically free and a couple years old.
Just do a Google search for college open courseware and start investigating. A parent can either take the course and teach the child or if the child is ready for it he can listen to the lectures and learn himself.
Years ago I started my astronomy lover on an Astronomy 101 course at a Michigan university. She loved it.
I searched and searched for options for her, but most curriculum I could find dealt with basic astronomy and she wanted more meat. I found a free video course so she was able to see the professor.
I don’t know much about astronomy, but I know how to find a resource to help my child learn. So my response to this public high school teacher would be to tell him that I don’t have to have my own water to fill my son’s cup.