"It's a Sunshne Day" is the song most associated with The Brady Kids, but was originally introduced on the parent series, The Brady Bunch, in January, 1973. Subsequently, the song was added to the cartoon soundtrack. Joramma20 uploaded this karaoke-style clip, which has the lyrics on the screen to make it easier for you to sing along. The clip ends with a portion of the show's open.
For what it's worth, and this has been pointed out to me by many on other boards, the poses for the Kids' musical numbers were redrawn from The Archie Show. You simply sub Greg for Archie, Jan for Veronica, Bobby for Jughead, and so on. Cindy is not an analogue for the Archies' mascot, Hot Dog, however. Hot Dog often had a baton so he could pretend to "conduct" the band, if you will, but that isn't the case here.