Swift_Mime_CharsetObserver | --Swift_Mime_MimeEntity
Located in File: /vendors/swiftMailer/classes/Swift/Mime/MimeEntity.php
Returns NULL if no body has been set.
These are not just the immediate children, but all children.
For most entities this will likely be the Content-ID, though it has no explicit semantic meaning and can be considered an identifier for programming logic purposes. If a Content-ID header is present, this value SHOULD match the value of the header.
The lower the value, the more outermost the entity will be nested.
This includes grandchildren.