In “Dune: Part Two,” Jessica is still pregnant with Alia and doesn’t actually give birth to her during the course of the film. However, Jessica can communicate with her unborn daughter with telepathy. Paul briefly has a vision of his (grown) sister guiding him through the desert, and it’s at that point we see Taylor-Joy’s cameo. Alia is not an ordinary unborn baby, either. Because Jessica drinks some mysterious sandworm extract known as the Water of Life (or worm juice, as I call it), the unborn child within Jessica is suddenly granted special powers.
In the “Dune” book, Alia is born with the mental abilities of a fully grown adult — she’s basically an adult in a child’s body. A child with this much power is usually killed by the Bene Gesserit, the manipulative religious order that Jessica belongs to. But Jessica, living among the Fremen, decides to keep the baby. As a result, Alia is labeled an Abomination. In the book, Alia grows to be about four years old (“Dune: Part Two” condenses the timeline to a matter of months instead of the years that span in the novel). During a big climactic battle in the novel, Alia even commands groups of soldiers and kills some people. Yep, she’s a deadly kid.
Alia has actually been depicted on screen before — she shows up in David Lynch’s “Dune,” played by Alicia Witt.