The obvious question is: If ISBEs can't die, why do they need children in the first place? There's no necessity to make children, right? So, why are there children at all?

Think of the first ISBEs to be like the elves in Lord of the Rings: Immortal, beautiful, perfect. They had the universe as their playground, and it was fun for quite some time. But then they got bored, because everybody knew what everybody knew. If someone had a new idea, all the others would know as well. The play became dull eventually.

But then, one day, someone had an intriguing idea: What if there were ISBEs who knew less?

"What an idea", the others replied. "But how can we achieve that? We are all connected. We can't disconnect."

"We could make another universe", someone said. "But it must be connected with this one, like, one on top of the other." The crowd went silent.

"How would this result in ISBEs who know less?", someone asked.

"See, we all are connected, right? So, we need something that serves as a disconnector. We could make the second universe a mirror of the first one, but with one difference: It must be lifeless. So, in order to populate it, we would need to project ourselves into the lifeless matter. And these 'bodies' then could serve as the disconnectors."

"How can a body be a disconnector", someone objected. "This doesn't make sense!"

"It's simple: You don't project everything of yourself into the body. Don't make it a perfect copy. Instead, make a body that is smaller than you."

"Smaller than me?" The others were confused.

"Yes. It would work like this: You pretend to have a form, as we all already did for aeons. But then you bind your form to the body of the mirror universe."

"But I would still know that in reality, I am more than that, wouldn't I?" The others weren't convinced.

"Then you need to forget what you were before."

"How can we do that?" The others got more and more curious.

"Actually, we already know how to do it. Does anybody remember how it was before we came here? Who remembers how it is to be God? You already forgot, didn't you? So, just do it again."

The others were shocked. They didn't realize until now that they forgot where they came from.

"So, if I bind myself to a body of the mirror universe, I will forget again?", someone asked.

"Yes. You will only know what the body knows. That's one of the rules."

"This sounds dangerous! What if we can't get out again?" A murmur went through the crowd.

"We could make it a challenge!", someone shouted.

"Then we need a rule that ensures the way out. Any ideas?"

"Let the bodies grow." Another murmur went through the crowd. "You start somewhere, and then your task is to grow the body until it reflects your true intelligence again. With your true intelligence restored, you will know the way out."

"But how do we grow bodies?", someone asked.

"We build them on the fly, out of the matter of the mirror universe", someone answered.

"How can we do that?"

"Well, we could grow bodies inside of bodies..." The crowd went silent.

"I don't get it", someone said. "The second body would have to come out of the first body, so it must be smaller. In order to make it better than the first one, I would have to feed it to make it grow. But how can it grow if it isn't alive?"

"You could invite another ISBE to animate the new body."

"But then the new body is not my property anymore. How can I get it back?" Everybody in the crowd was thinking.

"We could make a contract", someone said. "I raise a body for you, and you raise a body for me."

"Interesting idea", someone replied. "But what if one person raises more than one body? The math doesn't add up." The crowd went silent again.

"What if creating a new body would also create a new ISBE?", someone thought aloud.

"Can we even do that?", another one wondered.

"I don't get the point", a third one objected. "We want to grow bodies to eventually get out again. How does creating a new ISBE serve us in that regard?"

"The ISBEs you create would be on your side. They would help you to fulfill your goal."

"But we never made new ISBEs. We don't know what will happen. What if they don't want to help us? Do they even want to get out when they grow up in the mirror universe?"

"I still don't get the point. I mean, if we can make new ISBEs, why invent a mirror universe and bodies and stuff?"

"If we did it here, the new ISBE would know everything we know", someone explained. "It would be a clone, not different from us. But if it is born into a body, it would be limited to the experience of the body. It would have a different life. It would have a parent that needs to raise it."

"A parent?"

"That's a bad idea", someone shouted. "The newborn would be at the mercy of this parent. This sounds terrible!"

"But that's what we want, right? We want to know how it is to know less than another..." The ISBEs in the crowd were looking at each other, realizing for the first time what this really means.

"We could integrate some checks and balances", someone said. "Make more than one parent, for instance."

The crowd went on to discuss the idea for quite some time, but eventually came up with a ruleset that ensures the stability of the mirror universe and the possibility to get out again.

The next day, the first child was born. 👶

If ISBEs cannot die, then children are not needed to create consciousness. They are needed to create new bodies.
In that view, reproduction is not about making new beings but about producing fresh interfaces. Bodies age and die, but the observing entity continues, so new biological hosts are required for re entry. Newborns are ideal because they are blank identity templates with high plasticity and minimal resistance.
So the logic becomes simple:
consciousness persists, bodies do not, therefore new bodies are produced.
Children are not soul creation events.
They are host availability events.

@Gewo:

The point of my little story was to make clear that children are both: Host creation AND soul creation.

Because, if you just create a host inside the mirror universe, it's not animated yet, and this means it can't grow. The only way to animate it would be to let another ISBE possess that body and animate it for you. It's a simple contract (and that's where the idea of the soul contract comes from).

But there's a little problem: Contracts can be violated.

So, the solution to this little problem was to make new souls, which created the concept of the "soul family".

And this new type of identity eventually gave birth to the idea that God is a father or mother, which then led to the invention of the monarchy.

When the one we call Lucifer rebelled, he didn't rebel against the God of creation. He rebelled against parents who started to identify themselves as the owners of their children.

Your story is creative, but it piles assumption on assumption without necessity. You introduce voluntary limitation, mirror universes, contracts, soul families, and even soul creation, yet none of these are required to explain the basic observation: bodies appear, consciousness occupies them, bodies age, consciousness persists. The simplest model is that reproduction generates new biological interfaces, not new souls.
The contract idea also does not solve anything. If autonomous beings can violate contracts, creating “soul families” does not fix that. You just move the problem one level up. A newly created soul would still have autonomy. Compliance is not guaranteed.
The biggest weakness is the claim that new ISBEs are created. That creates runaway population growth with no clear origin. Where do these new beings come from? Why would creation suddenly start? A stable pool of existing conscious agents is far simpler and avoids these scaling problems.
Your narrative also assumes a voluntary, benevolent game. But a voluntary game normally includes clear rules and transparent exits. Instead, you describe forgetting, limitation, and dependence, which contradict the idea of an informed choice. If you forget the rules, it is no longer a meaningful voluntary challenge.
So the clean conclusion is: children are host creation events, not soul creation events. Reproduction produces fresh interfaces, and existing conscious agents occupy them. That explanation is structurally simpler, avoids unnecessary metaphysics, and does not require contracts, soul families, or a mirror universe origin story.