Kid had to be under 50 lbs, postage paid and usually rode in the mail car.
Edit for links:
https://postalmuseum.si.edu/researc...dest-parcelshttps://www.si.edu/newsdesk/snapsho...ial-deliveryhttps://www.smithsonianmag.com/smar...l-180959372/Quote:
Smithsonian National Postal Museum
June 13, 2021 ·
On June 13, 1920, the Post Office Department issued a regulation stating that children may not be sent by parcel post. U.S. parcel post service officially began in 1913. During the first few years of the service, some customers decided to test the limits on what could be mailed—which, for a time, included postmarking their children. This is a staged photo; in reality, children traveled with trusted postal workers (not in mailbags). The first child mailed in the U.S. was a boy in Ohio in 1913. It only cost 15 cents to send to him about a mile to his grandmother, but his parents did insure their child for $50.