An Extra on the Warner Archive Blu-ray of Mark of the Vampire.
The little girl's dreamland seems to be full of underwear: as the calico knight sets off on his quest, birds are seen nesting in the cups of a brassiere; the "bridge" across the stream is a corset; and the "castle" in which the Calico Dragon lives is a "union suit."
Filmed in the two-strip Technicolor process, hence the only colors are shades of green and red.
Happy Harmonies was a competitor to the Silly Symphonies put out by the Disney studios. Both were shown in cinemas as part of a film program. Many cartoons of this era thus has humor that appealed to both adults and children.
As mentioned, the dreamland incorporated undergarments of the time that surprisingly made it past the censors. It's possible the 'girl' was more grown up, along the lines of Betty Boop (who she resembled a bit), another cartoon character who featured in shorts during the era.