Well, nobody knows Iapetos, Koios, Krios ... important figures in a literary text called "theogony of Hesiod".
It was suspected, that Hesiod invented some of the related persons just to make his system run. Artificial gods, produced by a lonesome thinker to amuse the public. Perhaps this was reality, but bestseller have the creative function to invent "reality". Nobody knew Harry Potter some years ago and now you can watch him in cinema.
Let's proceed.
3 Cyclopes - no children
3 Hecatonchiren - no children
Just literary figures. No genealogical importance.
3 Cyclopes - they have only one eye at their forehead. This means "oneness", singularity.
3 Hekatonchiren - they've 100 arms and 50 heads. This means "plurality".
Together they mean "the "one and the many". They're a philosophical idea. Yin (many) and Yang (one).
And they mean 49 + 1. An old number idea, indicating the use of the mathematical scheme of the I-Ching.
Mythologicans of the past didn't understand this fine detail.
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1. Okeanos + Tethys - they get 3000 children, this expresses "limitlessness" and somehow "3".
The ocean - especially in the eyes of an old Greek - was a good idea for limitlessness. A great snake surrounding earth - water.
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2. Koios + Phoibe got 2 children, only daughters. It's the only Titans pair, that only produced "female elements".
One daughter is the "mild" Leto. She gets 2 children from Zeus: Apollo + Artemis, twins. In mythological history later Apollo replaces sun-god Helios and Artemis moon-goddess Selene. Leto is the first "young goddess" of the second generation (after Metis, goddess of wisdom, who occasionally is called a daughter of Oceanos), that got children from Zeus.
The second daughter is Asterie. She marries Perses, a son of Krios (4th position Titan) + Eurybia (the element-daughter of Pontos-Gaia with the probable function "fire", cause she has a "heart of flint-stone"). Perses has the function "to be wise".
The daughter of Perses/Asterie is the great goddess Hekate, the "old mother", the "old woman" and connected with dark things, but also with great success.
So we have from Koios/Phoibe somehow:
Hecate: the "very old moon" - never married
Artemis: the "young moon" - never married
Apollo: the literary prince of the Olymp - oracle and poetry - the young sun
(one shouldn't overlook, that Hecate is made from two parents - that means, she's the result of 2nd position and 4th position)
Well, Koios/Phoibe means somehow the region of the Moon. In 3 aspects:
young Moon: Artemis
young sun (young means: still hidden sun, )
old Moon: Hekate
Koios/Phoibe also means: "the female region" and "the "hidden prince". Princes are best hidden, when still in the womb of the mother, so this is also "female region".
Asterie, mother of Hecate, got the mythological destiny, that Zeus was eager to get her. She tried to escape and successfully she modified into the island Delos, somehow becoming unattractive.
Zeus found Leto, the "mild", instead, the other daughter of Koios/Phoibe. The pregnant Leto tried to find a place, where she could give birth to her twins. She found Delos, that means, the modified Asterie.
First Artemis was born and it took nine days, before Apollo got to the light of the world. This "9 nights" have a specific reason.
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3. Mnemosyne # Iapetos
Well, they didn't marry. But they are a pair, somehow. Mnemosyne had her function to serve as one of many partners for Zeus, and she very successfully got 9 children: The 9 Muses, and these 9 Muses did need 9 days to be born. And the Muses steadily accompany Apollo.
So one should look, how the poets arranged "mythological history":
Artemis born on Delos (Position 2)
It takes 9 days, before Apollo is born.
In 9 days the Muses are born (Position 3)
Apollo is born on Delos (Position 2)
Apollo is steadily accompanied by 9 Muses.
A nice (small) literary trick. The poets don't tell the context, but if one understands their thinking: this is a solution for the small riddle. They loved such things. Like authors of criminal stories. They don't tell you the background at the first page, you've to think yourself.
Iapetos: 4 sons from Asia .... I tell later more, but they are Atlas, Prometheus, Epimetheus and Menoitios. The basic idea of Iapetos is "humanity".
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4. Themis # Krios
Themis was also an object to the creativity of Zeus. Result: 3 goddesses, all relevant to the function of Themis, and that's Justice.
We know Themis and her 3 children still from the medieval Tarot ....
Yes, it's "Justice" and her 3 daughters are the other cardinal virtues "Prudentia", "Temperantia" and "Fortitudo" ... You miss Prudentia? ... Yes, that's a problem, a lot of people miss Prudentia, specifically in mythological questions ... :-)
Krios had 3 sons of Eurybia with the "flint-stone heart".
Perses, the "wise": He marries Asterie from 2nd position and gets Hecate, a "single daughter". So we learn, that Hecate has "wisdom", she's not only the "old Moon".
Asterios marries Eos and she's a daughter from position 5, Hyperion/Theia, which are responsible for sun (Helios) and moon (Selene) and the change between day and night (that's Eos).
Eos - responsible for "change" - "changes husbands". She's the prototype of the unsteady woman, a counterfigure to Aphrodite.
Asterios is only one of them. Others are Orion, but this stayed a little unfulfilled, a somewhat unclear Kleitos, Tithonos, who was once young and now very old but not dying and very specific is Kephalos - which means "head".
Asterios means "King of the stars". The children of Eos/Asterios are the winds and stars. Probably it was naturally for the winds in Greece to correlate with the change of the daytime.
The 3rd son, Pallas, married a dangerous woman, that's Styx, the river, which parts under and upper world. The 4 children are expressions of fight and strongness, one of them is Nike: "Victory".
The basic idea of the 4th position Themis/Krios is "motion" and "karma" or "causality" (which only exists with "motion"), also it is "fight" and also somehow "liberated sexuality" (which also is "motion" ... :-) ...).
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The 5th position is Hyperion/Theia and the kids are the different forms of light: Sun (Helios), Moon (Selene) and Change between day and night (Eos).
Not very difficult. The Tarot knows them: Sun-Moon-Star.
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The sixth position is the rest and the 6 children, 3 male and 3 female, is the base for the (later) Olympic community. Hera, Demeter, Hestia, Poseidon, Hades, Zeus.
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And all Titanic children together are a "Tarot" - a schematic system to interprete the world in an intelligent picturial manner.
It has 18 elements (Oceanic/Tethys present the limitlessness and are uncounted / children of Mnemosyne and Themis are part of this generation and the next, somehow they also stay "uncounted").
The Greek loved the 18, probably cause they had an alphabet with 18 elements, I already told that.