Just making guesses here but perhaps Kahleb was an unplanned pregnancy and all of the added financial & parental stress that goes along with caring & providing for 2 children was taken out on him? Or maybe Steve/Brad felt he was baby-trapped by Wendy/Pam and his feelings of resentment were misplaced?
Perhaps Kahleb's age/development level had some sort of significance? Maybe the parents were both horrible at dealing with children at his stage? (15-17 months?) Big enough to be running around and getting into things yet still too young to have much communication skills or understanding?
Spirited/Busy? Maybe Ryleigh was less curious, more easy-going and required less patience than the average child and Kahleb was a typical young tot or even a curious, busy, more high maintenance baby/young tot and they were very resentful of having to actually BE parents and keep a watchful eye on him?
The parents both had photos or posts about or including Ryleigh on their FB profiles, but barely anything about Kahleb. And Steven/Brad had a lot of posts even before Ryleighs birth either sharing ultrasound photos, info about prenatal appts, counting down the time until her arrival, pics after her birth, video of her first walking etc. He at least seemed genuinely excited about Ryleighs arrival and about being a new dad and his new little family. And again, barely anything about Kahleb. One post about him being due in 7 weeks, a more recent-ish photo of he & Ryleigh seated together on a push toy and then that (fabricated, imo) post about Kahleb's organs are growing but not his body "medical condition" announcement and that's it. I remember after looking over both parents FB pages early on and getting the gut feeling that this wasn't simply a matter of the "normal" 1st kid sometimes has every small milestone celebrated with more excitement than subsequent children. The stark contrast between the two children even just on Facebook seemed abnormal.