This is a photo from the National Postal Museum showing a child posing with a letter carrier in 1913. (photographer unidentified, Smithsonian Institute, Collection: U.S. Postal Employees)
At this time you could legally mail children. It was actually done several times. Postage stamps were attached to their clothing and the children rode with railway and city carriers to their destination.
When the Post Office Department found out this was occurring, they issued a regulation to prevent “the sending of children in the mail”.
There are definitely times when I’d like to mail mine away, but with today’s postal rates it would bankrupt me.