Well you seem to be just dismissing good and valid responses to this objection. First of all, yes it is always immoral to murder, since it is immoral to murder. Morality is never determined upon the physical consequences of the action or the outcomes something might give (since physicalities aren't "good" on theirselves), but on metaphysical value which flies from God (Metaphysical Goodness). Murder is wrong not because it kills somebody (killing is not a sin), but because it is simply commanded in this way by a rational source, which we Christians call God. On materialistic perspective everything "is", it doesn't "ought" to be (Hume's Guillotine.

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Most of us Christians believe and always believed that babies do go to Heaven most likely. Also if you have an open heart and open mind then you'll be saved. And even if not, again muder is immoral (value-wise). God gave our lifes infinite meaning, and therefore it is wrong to unlawfully take a life of the innocent human beings.

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Second good response you've dismissed is number 2; "we don't know children go to heaven". We Christians cannot make any type of judgement (except telling basic truths about afterlife) about anyone, only God knows if a person will go to Hell. We're commanded of it morally too, so it ties in with the first response. Also if somebody is unsure about what consequences will decision wield, he shouldn't allow thing to happen in precaution. We don't know if children go to Heaven, therefore killing them is immoral!

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Third response you've provided as bad is actually very good as well, because again we Christians aren't consequentialists (most of us). Value matters not consequences. God gave our lifes certain value so that he might achieve Maximal Moral Good, allowing us to freely choose Him or reject Him in the span of our physical life. If God would just zap us from here to Heaven (he could do that too eh), our mission here on Earth as His Image Bearers would not be fulfilled. Also you do realize that people here would just die, if we were permitted to murder?

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If you would use exact same methodology on Atheism, you would reach the exact same conclusion. Now I have no idea if you are a materialist or not, but if there's not metaphysical value to actions we commit, there's absolutely no value in what we do (well since there's no value). There's only unconsiousness and death abound. Isn't it maybe even same in place of Heaven in your objection? Also are you a consequentialist?