I was looking through the posts & I don't think I saw if the mom was taking any kind of meds for her OCD. If she really had it & she had it that bad, she needed some. I too have OCD & so do quite a few of my family members, though I have to admit none of us have the cleaning obsession. I can see that if you were that obsessed with your house, your kids would not be very well looked after. From my own personal & family experiences with OCD, we have been hyper-vigilant parents & caretakers because our OCD has taken the form of trying to prevent harm, so therefore we are overprotective, but I can imagine there are all sorts of different forms that OCD takes.
This could possibly be a case of harm coming to Sky through neglect. And if she cleaned constantly then there may have been a lot of toxic cleaning products within easy reach of a child. Not to mention if she was taking medication, he could have gotten into that. The mom sounds like she could potentially have a case of Post Partum Depression, considering her weird dreams of killing the kids. She could have caused bodily harm to Sky herself.
Unfortunately I feel as many on this thread feel, that there is a huge chance that Sky is no longer with us. I feel that he was never in the car at all that day and the auto breakdown was just a coverup. My other thought was that he really did get sick somehow through her neglect and died on the way to the hospital, if in fact she was ever actually headed to a hospital. She could have disposed of him along the way, except it would have been difficult to hide that fact from the daughter. Another thought is that or he was in the car & she had arranged for someone to take him, which would be a better scenario since he could hopefully still be alive and cared for by someone.