<div id="root"></div>

import React from 'https://cdn.skypack.dev/react';
import ReactDOM from 'https://cdn.skypack.dev/react-dom';

const {useState} = React;

function Parent(){
  const [parentCount,setParentCount] = useState(0);
  return <div>
    <p>Parent count: {parentCount}</p>
    <button onClick={()=>setParentCount(c=>c+1)}>Increment Parent</button>
    <Child parentCount={parentCount}/>
  </div>;
}

function Child({parentCount}){
 const [childCount,setChildCount] = useState(parentCount);
  return <div>
    <p>Child count: {childCount}</p>
    <button onClick={()=>setChildCount(c=>c+1)}>Increment Child</button>
  </div>
}

ReactDOM.render(<Parent/>,document.getElementById("root"))