What we're seeing is that Mike would prefer to not be Lizzie's primary caregiver given the choice.
Panel 1: Muppet Mouthed Michael yells "Mom! Lizzie's getting in to everything!"
Panel 2: He goes on to state to his Bug-Eyed Mother that he and Lawrence can't play without her gettin' into their stuff.
Panel 3: As he wipes his nose on his sleeve and SNIVELs, she, in a confused tone of voice, points out that he usually plays so well with his little sister.
Panel 4: As he tells her "Yeah, but that's when I don't have a choice", she frowns in disgusted anger.
Summary: That's because she'd rather not admit that she cannot after all force a six year old boy to watch over a toddler and have him like it. To admit that children would rather play with their peers than give her time to drink coffee, watch her stories, do meaningless busywork while letting mission-critical chores slide and grouse about how those people in Haiti think THEY have it tough would mean admitting that all the other stupid things she believes are wrong and she doesn't want to do that.
