Can't say I haven't threatened to do the same thing sometimes, but they always know you are bluffing!
It doesn't often happen, but if my two are arguing and fighting in the car, I feel that it ruins my concentration, so I refuse to drive! I find a safe place to stop and refuse to move until everyone has said sorry and are behaving themselves again. Considering that I am usually going somewhere for their pleasure rather than mine, it doesn't take long for it to sink in that they are either going to be late, or are wasting time that could be better spent at the park etc.
Sounds like she totally lost it and put her kids lives in danger - it is only right that they have been removed from her care.