Idea of making a lullaby from Lay of Leithian came to me after reading a Russian translation of “Beren and Luthien” book, where line “Down crumpled Orc, down Balrog proud” resembled “Sleep tired toys, books are asleep…” from a Russian children show. But making a lullaby for Orcs required changes to be made. So, baby-orc should be afraid of Lúthien. I've made a quick research to prove that scary lullabies are the thing in almost every culture.

It also reflects Tolkien's belief that Evil cannot create art or life:

The Shadow that bred them can only mock, it cannot make: not real new things of its own.
(Frodo to Sam about Orcs, LOTR, ROTK)

Melkor spent his spirit in envy and hate, until at last he could make nothing save in mockery of the thought of others…
(Silmarillion)

So, most of the text was taken directly from Lay of Leithian, but with lines and words inside of them re-arranged. Out of 500 words over 300 were “stolen”.