Absolutely he committed several crimes. Child endangerment. Accessory to murder. Neglect. Willful endangerment. Take your pick. A good prosecutor should be able to put him away for nearly as long as her.
He KNEW she was a whacko, and still willingly left his "beloved" kids in her care all the time. You can't have it both ways---if she is a danger, he should have pulled them. If she wasn't a danger, he wouldn't have noticed anything wrong and had her on every drug except Thorazine.
Hell, willful neglect alone would be enough to give him prison time. You CANNOT leave infants in the care of kookburgers, END OF STORY. If you do, it's on you, not them.