How do you think he got the children out?
spoilers:
I know it isn't shown but how do you think Landis got the children out of the house? I know the sliding door was unlock in the kitchen. I was thinking he used that door to take the children out of the house.
Somone pointed it out to me that his obsession with illusions might have been the clue to why he killed the children, to break the illusion that harm can come to your children even in a nice house where the family has money and a responsible babysitter.