Hmm...

[stoutfellow]

A Tree Grows in Brooklyn, Betty Smith (1943):

A new sled with a flower someone had dreamed up painted on it - a deep blue flower with bright green leaves - the ebony-black painted runners, the smooth steering bar made of hard wood and gleaming varnish over all! And the names painted on them! "Rosebud!" "Magnolia!" "Snow King!" "The Flyer!"

"Citizen Kane" came out in 1941.

Comments

[muckefuck.livejournal.com]

I assume "Rosebud" was just a popular name for a sled back in the 40s, like calling your bike "Beast".