children-parent-react.jsx

// children component
class Children extends Component {
  state = {
    text: "text children"
  }

  changeText = () => {
    this.setState({ text: "text from parent" })
  };

  render() {
    <div>{this.state.text}</div>
  }
}

// parent component
class Parent extends Component {
  
   changeText = () => {
     // execute method of children component
     this.childrenComponent.changeText();
   }
  
  render(){
    <div>
      <Children ref={ el => {this.childrenComponent = el;} } />
    </div>
  }
  
}