I found that the code worked fine when editing posts from the back end, but not when editing with a front-end Form, where at the time save_post (or post_updated) action runs the post meta data has not been updated yet.
So, if you need it to work for both front-end and back-end editing, we need to modify the code and test if we are submitting via a front-end form, and if so, get the updated field value from the global $_POST object.
So my updated code looks like this:
/**
* Update child posts with field from edited parent post
*/
add_action('save_post', 'tssupp_form_submit', 10, 3);
function tssupp_form_submit($post_id, $post, $update) {
// Edit these as required
$relationship_slug = 'project-task';
$custom_field_key = 'wpcf-due';
if ($post->post_type == 'project') {
// get child posts of this parent
$children = toolset_get_related_posts(
$post_id,
$relationship_slug,
array(
'query_by_role' => 'parent',
'limit' => 999,
'return' => 'post_id',
'role_to_return' => 'child',
)
);
// get the parent custom field value
$field_value = get_post_meta($post_id, $custom_field_key, true);
// if submitted via CRED form this needs overwriting
if (isset($_POST['_cred_cred_prefix_post_id'])) {
$field_value = $_POST[$custom_field_key]['datepicker'];
}
// update the children with this field value
foreach ($children as $key => $child) {
update_post_meta($child, $custom_field_key, $field_value);
}
}
}
There is one complication, namely that different field types populate $_POST a little differently.
So for a simple field such as a single line text field you can get the value directly, e.g. with $_POST['wpcf-my-field'].
In my sample code my custom field is a date field, and I would get the value with $_POST['wpcf-my-date']['datepicker'].
I'll leave it to you to determine if you can use the first simple case, depending on your field type.
