They are all dressed up in pearl buttons and sing "Supercalifragilisticexpialidocious". A lady in a magenta dress plays the tambourine, a short man in a black suit plays the bass drum, a tall man in a black suit plays the bones, another short man in a black suit plays a stringy click instrument that looks like a banjo with a cymbal attached to it, and a lady in a blue dress plays the violin. Only the short man on the bass drum speaks.
As said above, the band only appears during the end of the animated sequence where they perform the musical number "Supercalifragilisticexpialidocious". Throughout the sequence, they end up playing the song just as Mary Poppins and Bert sing along during the musical number just as the children Michael and Jane enjoy the performance as the rest of the animated characters dance along. At the end of the song, they are last seen leaving after the rain in the real world arrives, just as Mary, Bert, and the children leave the animated sequence.
The drummer and violinist also make a cameo appearance in 101 Dalmatians II: Patch's London Adventure: the violinist in a phone booth, and the drummer walking two Great Danes during the "Twilight Bark" scene.