1.) Khufu, the central figure in the story, had been seeking details pertaining the secret chambers of the sanctuary of Thoth - he wants to replicate 'such' -- in his own pyramid presumably -- yes?
2.) The dude who came highly recommended as able to provide him with 'such' declares his inability to do so, however, makes another recommendation...
3.) His cryptic onward referral is an unborn child - abstract in the sense whereby it is inferred/translated:
a.) The eldest -i.e. in the usual sense of coming forth from the womb, would equate to a first born - eldest right?
b.) Alternatively, the 'eldest' in the womb, could be the first conceived (time-stamped) - in the production run...
c.) Or even -- since you're implicating the sky -- order of ascension, or descent concerning three objects.
The question to consider therefore, is which one of the options can oblige the King's request - and how?
Elsewhere in the document, I recall, Khufu was reassured of the triplets posing no threat concerning succession of the throne by his son, the Prince and likewise, in turn, his - indicating the unborn trio are destined to follow after Khufu's grandson... Rather puzzling however: how can someone ('the eldest') provide Khufu with any information subsequent to expiry of his life term -i.e. what would be the use thereof, when his place of internment required preparation & completion in advance?
..the first child, Isis utters 'May you not be powerful in her womb, in this name of yours of Userref!’ User is a name for Osiris as established above in the discussion on Reusre. Osiris is also connected to the concept of power and of re-birth. As such, I suggest that Userref is a re-birthing of Osiris.
Now lets deal with the third child where Isis utters 'May you not stay dark in her womb, in this name of yours of Keku!’
There rather seems to be some confusion concerning 5th.Dyn. founders & rulers, involving pyramids from Saqqara to Abusir -- issues to be resolved by Egyptology -- regardless, I fail to see the point in the matter... What specifically, in context of aiding Khufus' quest, does your conclusion mean in that regard - please clarify where the 're-birthing of Osiris' features in your screen astronomy.