somewhat

mostly jeremy but tess is also important

The nursery rhyme you are referring to is "Hush Little Baby." The complete verse is: "Hush little baby, don't say a word, Papa's gonna buy you a mockingbird. And if that mockingbird won't sing, Papa's gonna buy you a diamond ring."

If that mocking bird don't sing

Mama's gonna buy you a diamond ring

I have to imagine that it refers to the popular lullaby. "Hush little baby, don't say a word Papa's/Mama's gonna to buy you a mockingbird And if that mockingbird don't sing, Papa/Mama's gonna buy you a diamond ring." The fact that the little girl in the movie is unable to speak seems to make this a very fitting title.