Well that would again be Mickey and Durante, in the 1934 MGM picture Hollywood Party.
But I'll let you off since that was just a short segment. The first true feature to combine live-action and animated characters in a major way was Disney's The Three Caballeros in 1944/45, 20 years pre-Poppins. Before this there had been films (The Reluctant Dragon, Victory Through Air Power, Saludos Amigos) that had included cartoon sequences with live-action, but not in combined interaction.
After Caballeros, Song Of The South in 46 really perfected the technique, Melody Time came along a year or two after, and we had MGM's Tom & Jerry pairings with Gene Kelly and Rather Williams, while even more extensive was the excellent half hour "Sinbad" sequence created for Gene Kelly's Invitation To The Dance, shot in 1954 but not released until the animation was finished in 1956!
All these came well before Poppins and arguably featured more interaction and/or animated screentime than Poppins' relatively brief Jolly Holiday sequence (at under 18 minutes, I believe).
So Poppins wasn't the first...not by a long-shot!
