I see your point. But in none of those examples, did the family members claim to be there when the child went missing. They lied ,after the fact, to try and throw off the investigations. [ well, MAYBE George lied about seeing Caylee leave with Casey that afternoon...idk]
It's usually a case of whether stories make sense after the fact.
Lisa Irwin's mother admits to being home, but she also admits to drinking and passing out.
Sky Metawala's mother said he was ill and she was taking him to the hospital - then she left him alone in the car by the side of the road, later claiming someone kidnapped him. (We don't know what happened there, but she did put herself at the scene just before he "disappeared.")
Kyron Horman's stepmother claimed she never took him out of school that day, but the teacher never saw him again after she left the building, and police seem to believe she drove away with him.
I personally believe that kidnappings by someone outside the family are quite rare. :cow:
In all my years on Websleuths, the only one that really surprised me was the Elizabeth Smart case, and that was partially explained by the layout of the house and the fact that the girls' room was away from everyone else's. Also, ES's upbringing made her very aquiescent and obedient so that she went along with the kidnappers. I honestly doubt most children, even a toddler, would go that quietly in the middle of the night when taken by a stranger.