test.html

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Fetch API Demo</title>
</head>

<body>
  <div class="container"></div>
  <script>
    const block_id = 'f553f410-7bef-45cd-b9dd-8af0fa7d6ea8';
    // console.log(`https://api.notion.com/v1/blocks/${block_id}/children`)
    fetch(`https://api.notion.com/v1/blocks/f553f410-7bef-45cd-b9dd-8af0fa7d6ea8/children`,
      {
        method: "GET",
        credentials: 'include',
        headers: {
          'Authorization': 'secret_632Cu9NwJzxefl8g1nv0tVKo8XT6FDOaEiYrHlLsRv3',
          'Content-Type': 'application/json',
          'Notion-Version': '2021-05-13'
        }
      }
    )
      .then(
        function (response) {
          if (response.status !== 200) {
            console.log('Looks like there was a problem. Status Code: ' +
              response.status);
            return;
          }

          // Examine the text in the response
          response.json().then(function (data) {
            console.log(data);
          });
        }
      )
      .catch(function (err) {
        console.log('Fetch Error :-S', err);
      });
  </script>
</body>

</html>