To be clear, you are advocating that the police leave a woman with a history of mental illness, brandishing two knives, while extremely agitated, alone with 3 children?

She was unstable, carrying sharp items and had young children with her - deescalating the situation by leaving just puts the kids at risk.

they can’t do that when she has potential victims inside the premise. If they left and she murdered her children they would be responsible. 

Cops are trained to shoot center mass and the fact is in a dangerous situation, a cop is more likely to miss their shot if they aim for limbs. It’s a much smaller target and stress from the situation is going to disrupt their aiming ability.