In 1775, the Continental Congress appointed Benjamin Franklin Postmaster General (PMG). Franklin had previously served in that post under the Crown. He was followed by his son-in-law, Richard Bache, and Ebenezer Hazard. The first Postmaster General of the new United States of America was Samuel Osgood.
Even the Oxford English Dictionary throws its hands up and admits “of the actual origin we have no evidence.” But, the OED continues, “the forms dada and tata, meaning ‘father,’ originating in infantile or childish speech, occur independently in many languages.” In other words, “dad” might come from baby babble and entered adult vocabulary from there as “dad,” but it’s not a sure thing.