I'm always so baffled. It seems like we're supposed to believe he was birthed recently by the hitcher who kills herself. But, while I'm not the best at guessing child ages, the kid seems more like a year old, at the very least way more than just a matter of days.
shareThe child that Erin rescues at the end?
I've always assumed that was the hitchhikers little sister/brother. The group find a family photo of the hitchhiker and her family;
https://i.postimg.cc/Cx4NR46c/Tcm5jar2.webp
The hitchhiker is at the back to the left, blurred. Presumably it's the hitchhikers mother that is holding the baby and is thus the parent.
From a fandom wiki;
Prior to the events of the film, the Hewitts attacked the family, presumably killing the parents and the son. They also appear to have robbed them and looted off photos of their victims as souvenirs. The teenage daughter of the family, Jessica Crawford, managed to escape and was picked up by a group of concert goers, but ended up breaking down in the wake of her family's death and shot herself in the back of Kemper's van. The only known survivor is the unnamed baby of the family, who was spared and temporarily adopted by Henrietta as a surrogate daughter, before being rescued by Erin.
Mr. Crawford (father)
Mrs. Crawford (mother)
Jessica Crawford (daughter)
Teenage Boy Crawford (son)
Baby Girl Crawford (daughter)