The reason she does this is that the dimwitted narcissist thinks that if her children were to suffer the consequences of stupidity, the other parents would think of her as being a bad parent and treat her like a pariah. What she doesn't know is that they probably actually think of her as that screeching failure of an enabler who let her slacker kids take advantage of her.
This, of course, is not the only way in which her obliviousness and need to be an enabler made her life worse. Let us take time to consider her habit of talking baby talk to Lizzie and inability to understand why the child took so long to sound like a normal child her age. Just as they expect Elly to storm in like a whirling dervish and solve all their problems for them, the kids think thatt children are supposed to sound a certain way. Why, the presence of a small child who speaks more adult English was enough to send one of them into hysterics.