Will billions make it to the Celestial Kingdom? Without being tested in mortality?

The question was put:

"Will billions make it to the Celestial Kingdom? Without being tested in mortality?
This is a topic I have been thinking about recently. D&C says that children who have died before reaching the age of accountability (eight years old) will be saved in the Celestial Kingdom. Any guesstimates on how many people that would be?

Here's mine: If you assume that roughly 70 billion people have lived on the earth (I know, a number that could vary widely depending on how long you believe the human race has been on the planet), and for most of human history, child mortality rates have ranged from 25% to even close to 50% (i'll use 35%), then approximately 25 billion people would have died so far and be guaranteed Celestial Glory. Judging by the percentage of the world's population that now accepts the gospel, it seems possible that the majority of people that make it to the Celestial Kingdom will have never been tested in mortality.

I'm curious to see how some of you reconcile this doctrine with the rest of the plan of salvation. If a key part of the plan is for us to be tested, how is it that so many will enter the Celetial Kingdom without being tested? Or do you believe that these children will at some point be tested?"

To which I responded:

I personally reject the "automatic Celestial Kingdom for any person lucky enough to die before turning 8" doctrine. In my opinion, the doctrine was probably originally developed to combat the prevailing erroneous doctrine of original sin whereby we are considered guilty before God without even having done anything wrong. This is evident when you look at the BoM and the commentary along the lines that those who consider little children to need baptism to save them from Hell are in the gall of bitterness. During the time that the BoM was produced, JS had a more limited understanding of heaven and hell. He probably imagined there were only two possibilities: eternal bliss in the Kingdom of God, and an awful fiery hell for everyone else. Joseph's understanding of different kingdoms and degrees of glory came later, but by then, the doctrine that little children who die get an automatic salvation pass had already developed.

In analyzing the question, I start from the premise that God is just. And being just, he would not subject one soul to a more difficult test to earn the right to Jesus' salvation than he would another soul. One way people try to reconcile the problem is by concluding that any child that dies before accountability was actually planned in advance by God to die, and that it was his will to take them because they did not actually need to be tested. I completely reject that notion for several reasons. For one, I can discern no meaningful statistical difference in the behavioral, mental, and psychological development of kids who die through some mishap or non-mental health disease and those kids who live on to the age of accountability. I would expect there to be an observable difference if in fact some kids did not need to live to accountability to be tested. Take, hypothetically, two kids. One is practically an angel who shows the utmost kindness in childhood, but later becomes a murderer as an adult, and another who is an absolute devil, exhibiting vicious propensities but dies at the age of 7 and 364 days in a car crash. How likely is it that the latter was "so valiant and/or spiritually advanced in the pre-existence" that he in fact didn't need to be tested at all, but that the former was the one who really needed to survive to adulthood so that he could have the opportunity to experience temptation and grow spiritually by choosing good? Extremely unlikely if you ask me; if that were true, there would seem to be no rhyme or reason to God's decisions, and he would be a respecter of persons and an arbitrary and capricious god.

Secondly, I cannot accept that the evil acts of man are always or even generally part of God's plan. Take, hypothetically, a parent who is afraid their child will live to the age of accountability, sin, not repent, and not go to heaven. In order to guarantee the child is saved, the parent murders the child at age 7. How do we analyze that? Do we say it was God's plan all along that the child not live to the age of accountability because he didn't really need to be tested? If so, can we really blame the parent for the murder if the parent was simply doing what God had planned all along? If, on the other hand, God had intended the child to live to adulthood and be tested, but the well-meaning murdering parent's actions thwarted God's plan, then wouldn't it be unfair to send some of the people who live to adulthood to hell (when if only they, too, had been killed as little children, they would have instead gone to heaven)?

I suppose there is one (and only one that I can think of) way for the "all little children go to heaven" doctrine to be true, and that would be that ALL of us will eventually end up in heaven anyway- it just may be a matter of time. But if that is the case, then we need to seriously revise our Mormon world view about at least some souls being cast off forever.