I believe the kids did eventually end up at the hospital, although Erin didn't really want to disturb them by taking them out of school. School is more important than impending, potential death apparently.

I think maybe Simon ended up bringing them in to be tested. She didn't actually give the same meal to her kids. She claimed she gave them "leftovers" on the Sunday night, that she'd scraped the mushrooms off, and then given them the meat. That was a lie. It appears her idea of "leftovers" seems to be the same type of meat, but it didn't comprise any part of the deadly lunch meal. There was no mushrooms on the meat to scrape off, it was a different piece altogether. That's why she wasn't concerned about her kids getting tested, because she knew what she gave them the night after was not tainted in any way. Why she chose to call them "leftovers", lie about scraping mushrooms off, then fail to show concern for her kids health, is anyone's guess. It fails to make sense. Unless she may have wanted it to look like many people ate the meal, but only about 50% of them appeared ill, so it was just bad luck and not a deliberate act to wipe out her in laws. Yet on the other hand, she herself claimed to have been sick as a result of the meal, meaning in her reality, 100% of those that ate the lunch were ill, but miraculously Erin barely was ill at all. In actual fact, I don't think anyone believes she was ill at all, it was all likely to be self induced. I'd like it all to make sense so we can wrap it up in a bow and call it done, but none of us think like her so it can't be done.