In reading the details of the case and the DCFS reports, there has been drug usage by, at least, the mother. The purchase of illegal narcotics, and I am not saying this is the case here because we simply don't know, typically involves characters that are not upstanding citizens or may have criminal or shady backgrounds (i.e. the sellers). If there were purchases of illegal narcotics that occur in or near the home, that may make the home a target, perhaps especially the innocent children involved.
I have also seen situations where drug usage occurs in a home setting that involves multiple people, often non-residents (people that come to join in the drug usage, sell, purchase that may not necessarily be that well known to the people in the house, sharing only the common bond that there is an addiction to the substance--it can be a revolving door). With the house looking like it may be somewhat insecure, I wonder if there isn't a possibility that a nefarious character that may have traveled in this drug circle would have seen the children and would have known access points to reach the child. For example, where I have live/lived, neighbors often can pinpoint the "drug houses," because there are usually people hanging out there at random hours of the day that are unfamiliar to neighbors, coming and going.
One theory may be that a predator gained access to the house through drug usage with designs on the children, and that predator would have been able to know the layout of the house or be able to incapacitate the parent(s) with narcotics. This may describe why the child is missing and does not, for what we know, appear to be in the home. If this occurred, the mother or father may not have even realized the child was taken until the next morning when coming out of their binge. In this case, I wonder if police are looking into known associates of the mother; neighborhood doorbell cameras and security cameras on the street would be a great way to identify all the vehicles that may have driven to the house or stopped within a distance of the house. I hope and assume police are on this and have requested this from neighbors.
This is just a theory and my speculation, but may fill some holes.