React - how to capture only parent's onClick event and not children

Since you are binding the handler to th you can use the currentTarget property. The target property refers to the element which dispatched the event.

_onHeaderClick(event) {
    event.preventDefault();
    console.log(event.currentTarget);
}

Check them

_onHeaderClick(event) {
   event.preventDefault();
   if(event.target === event.currentTarget) {
      // handle
   }
}

Like this?

event.target.parentNode