I rewrote the lyrics to Justin Bieber's song, "Baby" while I was teaching a 4th grade unit on the branches of the government. The song lyrics go through the roles of the executive branch, judicial branch, and legislative. My students came up with motions throughout the unit, and they loved dancing and singing the song, while learning.
It's easy to learn and fun to sing! Your class will enjoy it. Even children who aren't big Justin Bieber fans thought it was cool to be singing such a popular song in class, just with different lyrics.
I also have in my "store" the PowerPoint for the unit that goes with these lyrics (lyrics are included), as well as handouts for students as they take notes.