Yes, it was the most cruel action possible. But as a way out it was really stupid. At first place, it was dangerous to do to a man like Michael a thing that he could never forgive, he might easily kill her at that moment. If he did not, it was extreme humanity .
And of course she could not really think that he would let her separate his children from him. It was certain that she would lose her children, anyone would figure that beforehand. Senseless cruelty.