Hush-a-bye is a song performed by True in Season 1, episode 9 "The Kittynati." She sings it to a bloated puffy-puff trapped in a wall underwater in order to calm it down so it can deflate and become free. The Wish of the same name supplies the music as True sings.
Hush-a hush-a hush-a-bye
Don't be afraid, no need to cry
Think sweet thoughts then close your eyes
Go to sleep hush-a hush-a-bye
This song is sung to the tune of "Hush, Little Baby".