"and if the husband is a brutal wife beater who becomes a mass murderer...is the wife criminally negligent because she never told the cops he was mean?"
Not under the law, because you have no general legal duty to take affirmative steps to protect the public.
As a parent, however, you DO have an affirmative duty to protect your children.
(To give the classic example, LEGALLY, one can walk by a drowning stranger and do nothing (assuming you did not put him in the pool, the pool is not yours, or you're not the lifeguard, etc). However, if it is your child, you ARE responsible, and must make all reasonable efforts to save the child.)
Here, it's undisputed she was nuts. She had been in the looney bin several times because she was a "danger to herself and others." He knew this. He knew she was going downhill, and yet left his kids with her.
That's criminally negligent, just like leaving your daughter with a known sex offender or something similar.
Not that he doesn't deserve some blame...sure he does...but she drowned them...as hard as that is to accept...that a mom would murder her 5 children....brutally