Why won't my nested React components render?

To allow components to contain children and render them correctly, you have to use this.props.children. This is passed to all components with children as a prop and contains the children of the component, as explained by the React documentation:

Containment

Some components don't know their children ahead of time. This is especially common for components like Sidebar or Dialog that represent generic "boxes".

We recommend that such components use the special children prop to pass children elements directly into their output:

function FancyBorder(props) {
    return (
      <div className={'FancyBorder FancyBorder-' + props.color}>
        {props.children}
      </div>
    );
}

This lets other components pass arbitrary children to them by nesting the JSX

function WelcomeDialog() {
    return (
      <FancyBorder color="blue">
        <h1 className="Dialog-title">
          Welcome
        </h1>
        <p className="Dialog-message">
          Thank you for visiting our spacecraft!
        </p>
      </FancyBorder>
    );
}

As described in the documentation, some components don't know their children ahead of time -- they may be generic wrappers or boxes of content that vary, which is what your ControlPanel is. So, to render the children of your component, you must render the children from the children prop explicitly in the parent's render method. Thus, apply it like this to your code:

const ControlPanel = (props) => {
    return (
      <div className="control_panel" id="control_panel">
        {props.children}
      </div>
    );
}

Notice how props.children is rendered (not this.props.children because it is a function component).

You can access the nested elements through props. So in your case do this:

const ControlPanel = (props) => {
  return (
    <div className="control_panel" id="control_panel">
      {props.children}
    </div>
  );
}