Christians: Why don't you kill all children if all dead children supposedly go to heaven?

Because, by raising the children responsibly, we can prepare them for all that Satan can throw at them. When they can do just that, they can go out into the world and prepare others to become strong. They can go and give hope to the hopeless, mourn with those that mourn, and help heal the broken. Plus, we are on this Earth to prepare for Heaven, and it is absolutely necessary that we live this life, even if it's hard.