This is because time and again, Elly stupidly refuses to supervise Michael like she's supposed to because, as always, she thinks that letting her children run around like maniacs while she drinks her coffee and does her pointless busywork will teach them responsibility. Given that the only way that could happen is that if he doesn't do so, he'll die a gruesome, pointless and eminently preventable death, calling this a good idea is like pouring cream, sugar and salt on what comes out of a blast furnace and calling it ice cream.
That being said, let us sample Elly's iron ice cream. Her idiotic need to enforce 'fairness' by letting three-year old Lizzie have her fair share of time with a small creature she shouldn't be allowed access to sets the tone for the arc by having her act martyred because while her nine year old can see how God-damned stupid that is, the idea that a small child can't understand that a hamster cannot take man-handling doesn't occur to her in the least. He sees it as "Lizzie was just about to squish Humphrey to death because she doesn't know how fragile he is" while she sees it as "Michael is hogging the rodent to himself because he's bad." She also gets her arse in the air because John prevents Michael from introducing Humphrey to Farley to "see what happens". While her stupidly allowing Mike to let the poor thing rattle its brains falling downstairs in its exercise ball is pretty damned awful, the worst example of irresponsibility on her part is her nitwitted refusal to check if Humphrey's cage is secured thereby allowing the poor little creature to risk getting trod on or mauled to death or something.
The reason for all of this is the same reason that Elly stupidly lets Michael be Lizzie's primary caregiver in the first place. Elly is simply too self-absorbed and too ready to lament the fact that she has nooooooooooo help and noooooooo time to HERself to be bothered doing anything but whine about the work she doesn't freaking well actually do and which Michael cannot do. He can't keep Lizzie from drinking paint thinnner because she'd just yell and scream. He can't keep her from sticking a barrette in an electrical socket. He can't keep her from running into traffic. He can't do anything but Elly thinks that since she needs him to help out so that she can have HER life, he can do all of those things and more.
The real pay-off of this horrible celebration of animal abuse is why Elly and Lynn never want to deal with hamsters again: they object to cleaning up after them and having to make sure their idiot children don't end up killing them. That's right. All they learned is "hamster = poor, put-upon ME!!!!!" Ah, well. At least her need to not get criticized has made her give up on CT. That way, we no longer have to listen to all the other entitled shits who rant about the unfairness of how they can never possibly live the lives they had before they had children.