It's typical in cases of schitzo for people to become overly religiously zealous and lose perspective. Any LDS person who understood the doctrine, would not think it's a good idea to kill their kids. And a person who did not understand the doctrine would have to have stronger faith in that misconception than most people in the world have about anything. It would be such a huge gambit to sacrifice your mortal and eternal lives so your kids could go to heaven, that it would take absolute certainty to overcome the basic moral obstacles and fear of committing murder against the kids you love.
People who have absolute certainty about correct principles would only obtain it by having a sound mind and good understanding of the doctrine.
One of the most central tenets of Mormon theology is of the family unit being together for eternity. Another is that we are here on earth not just to obtain these physical bodies, but also to gain experience, learn to know good from evil and to establish families of our own. Killing your kids would deny your family of that eternal togetherness, and it would deny your kids the chance to have the mortal experience, which is valuable for their spiritual and familial development.