There will definitely come a time when your son or daughter wants to learn about a subject you know absolutely nothing about (and often have no desire to learn, either!)
Whether it is about an obscure type of dinosaur, or the real life of an 1800s author, chances are you will be able to teach your child all about it, without having any prior knowledge.
Never veto a subject your child is interested in learning about, simply because you are not familiar with it. If you veto too many topics, you can easily discourage your child from learning and from homeschooling.
Nowadays, you will often find a resource in your community or online that you can tap into for help. Also ask family and friends about topics they are familiar with, as you never know who might have the same love of that dinosaur or author. And children can easily do this research themselves, which you can make into a research project for credit.
And you can learn all about the subject along with your child. You don’t need to be an expert in everything your child learns, and learning about new things alongside your child can be interesting for you as well.