We all know what a lullaby is: it's a sweet and soothing song that is usually sung to a child to help her go to sleep. It's often rhythmic and soft, and its purpose is to make someone sleepy.
So is "Lullaby" a lullaby? Yes and no. It has a metrical basis and it rhymes, so it has that going for it. It even might sound soothing when you read it aloud. But for a poem that sounds soothing, its content is absolutely the opposite. This poem is a little obsessed with death – okay, a lot obsessed with it. We don't know about you, but we'd probably like to skip all the talk of graves right before sleepy-time. Even though the poem ends on a beautiful and warm note, it's not exactly your typical lullaby.
Also, it's worth mentioning that in its first publication, the poem is referred to as "XVIII" and identified in the table of contents by its first line ("Lay your sleeping head my love.") We prefer "Lullaby," definitely.