Forms in a table won't show
In Drupal 7, a theme function like yours receives an array containing the variables declared in hook_theme(). If you execute element_children($form) with the form you defined, you get an array containing the following values:


checkboxes
quantinty
price
remove
submit
checkout


The control statement if (isset($form['checkboxes'][$key])) { will not be executed, if not in the case you have $form['checkboxes']['checkboxes'], etc.

Your theme function should use the following code.

function theme_cart_page_form($variables) {
  $form = $variables['form'];
  $rows = array();

  foreach(element_children($form['checkboxes']) as $key) {
    $row = array();
    $row[] = array('data' =&gt; drupal_render($form['checkboxes'][$key])); 
    $row[] = array('data' =&gt;drupal_render($form['quantinty'][$key]));
    $row[] = array('data' =&gt;drupal_render($form['price'][$key]));
    $rows[] = $row;
  }

  $header = array(
    'Checkboxes',
    'Title',
    'Quantity',
  );

  $output = theme('table', array('header' =&gt; $header, 'rows' =&gt; $rows));
  $output .= drupal_render_children($form); // Render rest of form components.

  return $output;
}


Actually, you should declare the theme function using the following code.

function yalublurudn_theme() {
  return array(
    'cart_page_form' =&gt; array('render element' =&gt; 'form'),
  );
}