Ooohhh... yeah, the thing with Chekov seeing the baby is included in the novelization of Wrath of Khan. When I read it I was like "what...?" I mean the author didn't follow it up with anything at all other them them finding the baby then getting found by Khan & Co, and readers are left to assume that the baby was taken on board the Reliant and I guess killed when the ship was destroyed. Niiiiice.
As has been pointed out, Khan's child didn't go anywhere story-wise, so it was easy to cut. However, as the novelization goes, Vonda McIntyre would have been working from the script, and not knowing that the scene where Chekov sees the baby was cut in the editing room, she would have put it into her manuscript. That the later scene, with the child in the transporter room, didn't make the novelization is suggestive that it might not have been in the script and was something Nick Meyer decided on during filming, perhaps to give the baby
some purpose narratively. Of course, it didn't work, and the scenes were excised entirely.