Enable a default value for checkbox groups

pewc_default_field_value_checkbox_group.php

<?php
/**
 * Enable a default value for checkbox groups.
 * Your default value must be an array.
 * You'll need to change the value of the $field_id to match your own Field ID
 * @param	$value			The current value of the field
 * @param $id					The field ID in the format pewc_group_888_999
 * @param $field			The field object
 * @param	$posted 		The $_POST object
 */
function prefix_default_field_value( $value, $id, $field, $posted ) {

	// Change this to the ID of the field you are setting a default for
	$field_id = 999;

	// Don't overwrite a value that's already there
	if( $value ) {
		return $value;
	}

	// Now look for our Field ID
	if( $field_id == $field['field_id'] ) {

		// You need to enter the default values here, copied exactly from your checkbox group options
		$value = array(
			'My first default option',
			'My second default option'
		);

	}

	return $value;

}
add_filter( 'pewc_default_field_value', 'prefix_default_field_value', 10, 4 );