Green’s Dictionary of Slang

(US) a general term of address, in fact the audience may be all male.

1921		H.C. Witwer Leather Pushers 8: Now, boys and girls, the next ex-e-bition bout of the evenin’ is Kid Roberts [...] vs. Battlin’ Fate.
1930		Graham & Graham Queer People 22: ‘Boys and girls,’ Vance called, waving his arms like a prize-fight announcer, ‘want you to meet my old friend, Mr. White, from the Examiner.’.
1942		R. Chandler High Window (1964) 365: Was sapped with something hard before being shot. Likely with a gun butt. All that mean anything to you boys and girls?