One of the reasons why the Brown children never try anything more severe than pretending to have the measles around Nanny McPhee to drive her off, is that they were probably concerned about her making it a reality. Them pretending to have eaten Baby Aggie, like they did to drive away Nanny Whetstone, could well have had Nanny McPhee make it so they had literally eaten Baby Aggie.
Tying into Fridge Brilliance: The kitchen scene where Nanny McPhee forces the children to keep playing and nearly catapult their baby sister into boiling water:
Nanny McPhee would never have put the children in harm, her primary purpose is to help the families she's assigned to. Aggie was never in any actual danger. Other than the surface-level lesson she teaches about the importance of "please", she was showing them what would've happened if the children kept the "out-of-control" behaviour they did: the children would've hurt the baby themselves eventually.
A little bit of Fridge Horror I remembered is how, if Mr. Brown didn't get remarried and if the money ran out, some of the kids would be sent to factory to work, which is made worse because of child labor standards back then.
