https://facts.usps.com/sending-kids-in-the-mail/ sez,

Do not try to ship your kids!! In the early days of Parcel Post, a few parents managed to mail their children to relatives. In 1913, an 8-month-old baby in Ohio was mailed by his parents to his grandmother, who lived a few miles away. The baby was safely delivered! Regulations were quickly established to prevent any additional mailing of children through the U.S. Mail.