This term is used throughout the novel to refer to the creatures who are "born" through the bite of a vampire, becoming undead themselves. It emphasizes their connection to the night and their shared, sinister nature.
The phrase appears most notably in the famous line spoken by Dracula himself: "I am the night!" and "I am the darkness!" and later "I am the Children of the Night!"
It's important to remember that while the term "Children of the Night" directly refers to vampires in Dracula, it has become a broader, more metaphorical term used to describe creatures of the night in other works of fiction.