An early reference:

"It's 'Napalm Stickes To Kids' On Skytroopers' Hit Parade" by John E. Woodruff. pg A8, Baltimore Sun, June 15, 1970.


See here: https://baltimoresun.newspapers.com/search/?query=napalm%20sticks%20to%20kids


Lydia Fish in her "Folksongs of Americans in the Vietnam War: Bibliography" has this incorrectly listed as

Woodruff, John E. "Songs Helicopter Pilots Sing on Flag Day in Vietnam: 'Napalm Sticks to Kids.'" Baltimore Sun, June 15, 1970.