Whistle while you work!
Hilter is a jerk!
Mussolini bit his weenie
now it doesn't work
Well, we can SORT OF date this one - it goes back no further than the 1930s, when "Whistle While You Work" came out. Ever since then, it's been one of a large number of Hitler-related schoolyard songs. The other goes like:
Hitler went to war
he lost his pants in the middle of france
in 1944 (and so on, with other years).
Since they started showing up in books collecting these things in the 1950s, folklorists have all asked the same question: do these kids singing the song even know who Hitler was? I'd say most of them over the age of 8 have at least a general idea that Hitler was a guy we fought in a war, though I doubt one in 20 could tell you who Mussolini was by the 1970s. There are plenty of similar phenomema; "Charlie Chaplin" and "Shirley Temple" rhymes were common on playgrounds well after the careers of Chaplin and Temple had wound down to a point where kids had never seen their movies, and Davy Crocket parodies greatly outlived the Davy Crocket craze of the mid 1950s that inspired millions of otherwise rational kids to go around wearing coonskin caps.