She/he(?) drugged/sedated the kids so they could have a quite dinner, because they couldnt find someone to watch them. The kid od's and dies. Hides the kid, rents a car 25 days later and disposes of the body.
Very plausable.
Even if it didn't happen that way, what kind of parents leave kids that young at home alone to go to dinner? I could understand if it was some poor family that had to work midnight shift and couldnt find a babysitter, but for dinner?