There are simple reasons why both Wallace and the old Tyrell replicants want the child: Wallace wants to be able to produce slaves faster than he can manufacture them, so those replicant slaves must be able to reproduce. It also bothers him that Tyrell knew more about bioengineering than Wallace. The replicants want the child because it is a sign that they are "real" people, despite being synthetic. The child is therefore a sign of hope. It is also a matter of keeping the child and the free replicants safe and free from anyone who would exploit them. They are escaped slaves, fighting for the freedom of all of the enslaved.
The Blade Runner movies make extensive references to Gnosticism, an early Christian heresy, as well as Milton's Paradise Lost. Philip K. Dick, the author of BR's source material, and Ridley Scott share an interest in this material.
Dick went beyond interest: in February and March of 1974, he experienced a series of visions that he spent the rest of his life trying to explain in his fiction and personal papers. Among the other fascinating information conveyed in these visions was the idea that modern times were an illusion, covering the fact that we are all living in a time loop during the Roman Empire during the time of the Book of Acts. We are, he said, trapped in the Black Iron Prison of this false reality. There is, however, someone trying to rescue us, who he called variously VALIS, Zebra, or the Plasmate, and who he identified as the symbiotic divine entity Christ who merged with the man Jesus.
The original Gnostics believed that Creation occurred due to the fall of an emanation of God called Sophia or Wisdom from a higher existence, called the Pleroma. Sophia, in her desperation, gave birth to the Demiurge, a blind, evil, sometimes lion-headed god/Devil who believed himself to be supreme, and who in turn created the physical world, trapping Sophia and the sparks of true Divine power that are human souls. However, the Demiurge lacked access to the true power of creation, and could neither create nor destroy souls within the created world, only sterile matter and lesser demonic spirits. The souls yearned to return to God in the Pleroma, and would come to escape the prison of the world through gnosis or knowledge of the Divine truth, which is actually a process of anamnesis or unforgetting.
In Blade Runner 2049, food and human substitutes are produced by Wallace, though the land (and the replicants) remain sterile. Wallace, like the Demiurge Ialdabaoth, is blind, leonine, a Devil who believes himself to be the new God. His name sounds like walls, the domain of Saturn/Cronus, the dark and grim father of Zeus, and the symbol of the prison. He creates beings he refers to as "angels" who cannot reproduce as humans do, and which may not have human free will or whose will is constrained (emprisoned), like Luv, who weeps at evil but serves loyally and cruelly nonetheless.
The Tyrell pyramid in the first Blade Runner, representing the Temple of the Presence, God on Earth, full of light:
It has been assimilated by the Saturnine Wallace Corp (background), imprisoning the darkened pyramid in 2049:
Deckard and Rachael, from the first film, produced a child, who might be K.
K is also "Joshi's Dog”. Does Joshi's Dog have Buddha nature, as the koan goes? Is he a real boy? He certainly has someone's real memory.
Among the other problems, most of humanity's records have been lost. Perhaps Ana Stelline, a memory artist with a celestial name, herself imprisoned in a sterile, though light-filled enclosure, can help K and the others un-forget. Can Sapper, Freysa, and the other old replicants (fallen angels) protect the child and redeem the fallen world? Who knows.