I don't either. But maybe the parents would rather not kill their children but rather let them live out a long and fruitful life and leave it up to their own decisions about whether or not they want to go to hell via sin rather than remove the choice from their hands. It's up the kids when they're adults and if you love your kids you'd let them make that choice and sleep in the bed that they made. Killing them removes that choice. Doing something bad is logically worse than letting something bad take its natural course... :/