A transaction will be automatically wrapped around every service builder method that starts with add, update, delete (and I believe create and some others, but don't have a list handy).
To bind all of these into a single transaction, the key is to create one entry point in your XxxLocalServiceImpl class to handle the bulk insert/update:
public Parent addParentAndChildren(final Parent parent, final List<child> children) {
// persist the parent object the way you normally would
// handle the children here, whether you do a simple case of deleting the current children and blindly adding the children in the list
// or a more complex method of selectively adding, removing, and updating children as given in the list.
// return the parent instance, this is standard for all add/update operations of Service Builder
return parent;
}</child>
Now when you call ParentLocalServiceUtil.addParentAndChildren(myParent, myChildren), this will all be handled within the scope of a single transaction.
So my original statement of "parent and children must be inserted manually" is still true, it is still up to the developer to manage the inserts.
