Elect infants, dying in infancy, are regenerated, and saved by Christ, through the Spirit, who worketh when, and where, and how he pleaseth: so also are all other elect persons who are incapable of being outwardly called by the ministry of the Word (WCF 10:3).
i) From time to time I discuss the question of the salvation of those who die before the age of reason. I mention that presumably they continue to mature psychologically in the afterlife. Now I'd like to flesh out the "logistics" of how that might occur. Obviously, what I say will be speculative. However, the speculation is an extension of things we know.
ii) In this post I'll confine myself to heaven for those who die before the age of reason. By "heaven", I mean the intermediate state for the saints. A disembodied condition for those dying in a state of grace.
iii) The Bible contains visions of heaven. Now, these may be symbolic, so that doesn't necessarily tell us what heaven is really like. Perhaps, though, the question of what heaven is really like might be the wrong way to frame the issue. It's like asking what a dream is really like. To take a comparison, consider the Colonial or Antebellum squares in Savanna, Georgia. We can say what these are really like because they are physical spaces with physical objects (e.g. trees, buildings). They have an objective, durable subsistence. Trees and buildings are located in relation to each other in fixed positions.
By contrast, I think heaven is like a very vivid, inspired collective dream. A dream has a simulated setting (dreamscape). The dreamer has a simulated body. Other characters in the dream have simulated bodies.
By the same token, people in heaven can have simulated bodies. And that's consistent with heavenly visions in Scripture. Likewise, heaven can have a simulated landscape, or cityscape, or seascape, &c. Heaven can be compartmentalized into a variety of different settings. There's no one way it has to be.
iv) Apropos (iii), children die in different historical periods. They die in different countries and ecoregions. Some live around mountains, or rivers, or lakes, or oceans, or jungles, or forests, or deserts, or cities, or villages, &c. Some died in the Ice Age. Some died in the ancient Near East. Some died in the Middle Ages. Some died in the 20C. And so on.
v) Let's pick an age group out of thin air for illustrative purposes: say children between 5-10 years of age. Let's say they go to heaven when they die.
If heaven has simulated spaces and places, they might to go a "part" of heaven that resembles the time and place they're familiar with. If, however, they grew up in a slum (to take one example), they'd go to a much nicer place. Maybe urban or rural.
Or childreen might go to a playground or amusement park. Or a meadow. They might live in simulated houses. There might be simulated wild animals as well as simulated pet dogs and cats and horses and whatever. The possibilities are endless.
vi) Children in heaven might be grouped according to age, language, culture, and ethnicity. At least initially. By that I mean, suppose you had pre-Columbian children who lived and died in the Amazon River basin. Maybe in heaven they are grouped together because they have so much in common, which eases the transition. That makes it less initially disorienting. But as they mature, they can branch out to explore other parts of heaven. Meet other kids (now teenagers) from different times and places.
Or maybe communication is telepathic, so they don't need to speak the same language.
vii) Heaven is full of men and women who died as adults. Men and women who were parents and grandparents in this life. They could be foster parents to the children. Not only do they have experience in child-rearing, but in heaven they are sinless. They aren't under the stress of life in a fallen world. So they could do a better job of parenting than they did in this life.
On this view, children could mature very normally, because their (simulated) physical and social environment is similar to what they knew before they died, only so much better.
In the case of children who had a Christian parent or parents, they will be reunited with their parents when their parents die. But at that point they will be grown children.
viii) Maybe children in heaven interact with angels. In addition, perhaps they get to meet Jesus or even see him on a regular basis. Although Jesus is physical, he can interface with disembodied souls the way a dreamer has a simulated body that enables him to interact with the dreamscape or dream characters. And because it's simulated space, he can be in two or more places at once.
ix) Their education could be individualized in a way that isn't feasible on earth.
x) Perhaps they can do things in heaven, like flying, that we can only do in dreams. Likewise, skindiving without having to breathe.
xi) On earth, children pass through adolescence. Hormones not only change them physically, but psychologically. Will there be something analogous to that in heaven? Hard to say. Perhaps that awaits the resurrection of the body.