We have several "modified" and personalized songs that we sing.
One is "The Heart Song," which is actually a Christmas carol that I sung in high school with my choir. I took a refrain from it and modified it into a lullaby when DS was a baby and around age 2, he named it "The Heart Song." (I believe the song is actually "Candlelight, Angel Light.")
We also do the "Tree Song," which is Rockabye Baby (where Mama catches baby at the end instead of baby falling), the "Star Song" which is, of course, Twinkle, Twinkle... and then the "Firefly Song," which is exactly that. The theme song to the sci-fi show, Firefly. Recently, DD has learned "You Are My Sunshine" and now sings that one to me.
We also say the "Monster Words" to scare off the monsters, but that comes after the songs, LOL.