Testing a private function in React Class using Enzyme

I am not able to test a function defined in parent class and invoked in child class. This is the link to my code- https://codesandbox.io/s/testing-parent-child-enzyme-stx7el?file=/src/App.js

This is my test code-

it('testing CTA btn click', () => {
   const component = mount(<Parent {...parentProps}/>);
   const element = component.findByProps({ className: 'cta-btn' })[0];
   element.props.onClick();
   setTimeout(()=> {
     expect(element.props.btnClick).toHaveBeenCalled();
   },1000);
})

But I am getting error as

No instances found with props {"className":"cta-btn"}

Please help.