I know this might sound silly, but the reason I think she died in an accident, is the fact they claim they left the apartment door open.
Now I don't have children but I wouldn't even leave my dog in a open house, I wouldn't even leave the dog in a house with a window open that was big enough that someone could reach in and open anything.
I apparently have more respect for safety of my pets that these two do for their children.
Its inconceivable to me that you would leave two children in an apartment that they could accidentally escape but also go out to dinner half away across the complex, so much so that I think its a lie.