Although LE has not ruled out the parents (or anyone else for that matter), one thing continually sticks out in my mind.
If LE thought for one second that the Celis' had any issues (drugs, neglect, abuse) that caused Isabel to go missing, you can bet that CPS would be doing an investigation. That is something that I have no doubt in my mind that we would have heard about by now.
Also, it has been mentioned by LE that the Celis family has a very active social life. IMO, a 10-year-old with an active social life and possibly a little more mature and confident than your average 10-year old. I think he would be perfectly capable of calling 911. My cousin's son called 911 when he was 5. His dad went into a diabetic attack and he couldn't wake him up. He was there alone with dad, and baby sister while mom was at work.