I speculate that Tabitha, Boyd, and Jade have been caught in a cycle of saving this town repeatedly. They might even be original inhabitants from when the town was created. At the end of season three, Tabitha mentions that she is Miranda and Jade is Christopher, and they had a child among those sacrificed children. However, based on the known timeline, this seems impossible, as the sacrifices seem to have happened long ago (likely at the time the town was established), and Victor's sister has already been buried (as seen in season three). So I suspect that Tabitha, in her extreme inner turmoil, made a misleading statement.
I've made a guess about this statement and combined it with Fatima's mention of the origin of the Smiling Man to hypothesize the broader context of the story. Tabitha and Jade have likely reincarnated into the town multiple times; their previous identities were Christopher and Miranda. Regarding them having a child who was sacrificed, I speculate that they were among the original inhabitants who participated in the sacrifices. As Fatima said, "It" might refer to the man in yellow (the demon) who lured the original inhabitants into sacrificing their children in exchange for immortality, thereby constructing this space. The original inhabitants were also transformed into tormenting monsters, possibly to harvest negative human emotions or for other reasons.
However, two forces oppose him: the sacrificed children and the boy in white (perhaps an angel? This would make the show somewhat akin to Faust). During the sacrifice, the corresponding original inhabitants of Tabitha and Jade may have resisted but failed, leading to their selection as reincarnators to participate in the town's salvation repeatedly. This explains why Tabitha and Jade are both Miranda and Christopher and have a sacrificed child because these are experiences from their numerous attempts to save the town.
After the demon built the town, he continuously recruits new people into the town, tortures them, and feeds off their despair to strengthen himself. The two reincarnators follow the newcomers into the town each time, trying to save it. Upon failing, their souls are transmitted out of the town and reborn in another child at some point in time (note that this point in time isn't necessarily later than their last failure, given that there are Storywalkers, time travel isn't entirely unacceptable either, which can explain the age discrepancy between Christopher/Miranda and Tabitha/Jade).
The demon clearly wouldn't allow things to happen so simply; he uses some means to make the reincarnators lose all memories of the town and their past lives. This is why, when Jade plays the violin and they remember their past experiences, the demon immediately becomes furious and appears.
Expanding on this hypothesis, we notice that in the town, Jade hallucinates seeing three people: Tom (the bartender), Christopher, and a Civil War-era soldier. Why does he see these people? Since Christopher is his past life, could Tom and the Civil War-era soldier also be his former identities? If this assumption holds, then there's someone else in town who sees people from the past—Boyd. Does this mean Boyd is also a reincarnator? Especially when Boyd is about to torture Elgin, and the priest repeatedly tries to dissuade him, could this be past consciousness advising his present self? These speculations don't answer all questions but seem to explain the mysteries surrounding Tabitha and Jade.
