First off, Marian had twigged to not being able to ask Elly "Why didn't you tell Lizzie how not to do that chore?" because she'd long ago figured out that her child never quite outgrew the assumption that everyone in the world saw the world precisely as she did. This made Marian afraid because she could foresee something others could not: Elly assuming that "Well, since I wouldn't drink weed killer/play around a river when there's a flood warning/stick a barrette in a wall socket/eat something handed to me by someone who really wanted to see what happened, my child would not either because s/he has my understanding of events" leading to an avoidable calamity.
Secondly, she had long since learned not to ask Elly why she didn't show her how to do the thing because that led to endless whining about how that would mean having to do the chore herself thereby somehow erasing her personality and turning her into a grinning robot without a will of her own who could only care about the unimportant and useless things children wasted their lives with.
Finally, there is one question that was clearly too dangerous to be asked: "Are you setting your children up to fail on purpose just so you can be angry with them because you think that's what being a mother is?" This is because she remembers something that would drive Elly into a big, screaming fit: Elly isn't aware of what she actually believes. Marian knows that Elly assumes sight-unseen that her children are all always-chaotic evil monsters trying to destroy her while at the same time believing herself to be an open-minded parent who believes in the innocence of her children. She can't tell her child that she's a doubleplusgood doublethinker without getting mauled.