As of our Spring '18 release, we have a single option for adding previously unpublished child items to the Affected Items tab of a Change.
From the Affected Items tab, click on "Add Unpublished Children.
Propel will look at each of the affected items to see if there are any previously unpublished child items and add them to the change, so that you will not be blocked from releasing a parent item with unpublished children.
A list of any child items that are already on other changes will display. You will have to either release the other changes before this change, or remove the items from the other changes and click "Add Unpublished Children" on this change again.
You can also "Add never before published children to Change Order (up to 400)" from the To Publish dialog when publishing an Item and adding it to a Change.