What most bothers me is that John and Elly do not understand that they are mostly to blame for this. What seems to have escaped their attention is that small children learn to become members of society and what their place in this world is by taking their cue from their parents. This blindspot is something that, with your indulgence, I'm about to explore in depth.
First off, let's take a look at how reluctant both of them are to be around their children when the going gets tough. When a child is looking for reassurance, you can count on John and Elly to be very reluctant to provide it. When a child simply wants his or her presence to be acknowledged, he or she is met with disdain on John's part and hostility on Elly's. What I find grimly amusing is that John and Elly make it quite clear that they'd rather not have Lizzie underfoot and spreading chaos on the one hand and cluck about how odd it is that she feels isolated on the other.
It's almost as amusing as how they both witlessly pit the children against each other lest they team up and take over the house and then complain about how the children fight one another. John and Elly are too stupid to realize that their inability to see how Mike's fear that he's going to be put out on the curb with all the other old trash they no longer need is most of why he drops unsubtle hints about how Lizzie is an interloper in her own home. To them, it's just something kids do and adult behaviour has no influence on it. To think otherwise would require them to do something that they find almost as humiliating as the prospect of admitting error: paying attention to what's happening around them.