You can do it simply without dealing with Typescript Interfaces, just while defining the function that is going to accept those props using '?' operator as follows

const ChildComponent: React.FC<{ firstName: string, lastName?: string}> = ({ fName, lName }) => {

}

Calling child component with LastName

<ChildComponent firstName={"John"} lastName={"Doe"}/>  

Calling child component without LastName

<ChildComponent firstName={"John"}/>  

Happy Coding! 🖤🐳