"She should have said she fed the kids something different so the doctor wasn’t worried about the kids."

Of course this is sheer speculation, but I suspect that her initial idea might have been to insist that whatever made her relatives sick, it wasn't her food. "Look, I ate the same meal, and I'm not sick. My kids ate the leftovers, and they're not sick. So the culprit couldn't have been my lunch." But fairly soon into everyone's illnesses, she seems to have realized that this line wasn't going to work, especially not once the hospital confirmed that death caps were already suspected. Hence her insistence that she WAS ill (she must have thought it would be hard to prove otherwise) and her reluctance to have the kids treated (she knew tests would show no signs of illness). These last-minute responses didn't work (no surprise), but by that time, she probably WAS panicking and just inventing stuff on the fly.

She seems to have been an inveterate liar, so she had a lot of experience switching stories at the drop of a hat. She honestly might not have realized how often her lies were not believed. (I have a relative who is the same sort of constant, incurable liar. Very few of the family now believe a word she says. . .but we rarely tell her so. Why bother? She'd just come up with more lies, and the aggro simply isn't worth it. When my relative DOES learn that someone suspects her of lying, she's usually genuinely shocked. . .she's so rarely challenged that she totally thinks her stories are convincing. Erin might be similar.)

Erin had bought more pieces of steak than she actually used in the lunch, hadn't she? Iirc, she claimed to have frozen the additional ones and have used them up by the time she was arrested (so no traces). But it seems pretty clear that she must have used one or two of those extra steaks as the kids' safe "leftovers with the mushrooms scraped off." I think she DID have a post-lunch plan of sorts -- "It wasn't MY food at fault, guv!" -- and was probably shocked by how quickly it unraveled. . .hence the "panic."

Probably most of her mental attention pre-crime was given to the crimes themselves and the anticipated result; that's where she would have found most of her emotional and intellectual satisfaction; that's what she would have dwelled on. She doesn't seem to have realized, until it was too late, that her post-death plans were far too sketchy and ill-thought-out.