I'm not sure I would say they are unaware of Hydaelyn, simply that they don't make as direct a reference to her (the song that is supposed to be from Rav's point of view does mention her). Bismarck...well he never talks to us (unlike pretty much every other primal) so we don't know (and he may be simply more beast like, in that he just operates on urges like the need to feed).
That said, I'm not sure the Children referenced are the primals, largely because both Igeyorhm and Tiamat talk about how they are creations of the mortal races - the Ascian calls them "the embodiment of mortal will - of mortal desire" while Tiamat says "the beings thy kind call gods are but specters of thy mind's creation, given form at great cost to Hydaelyn."
Not exactly a ringing endorsement of primals in general being original beings sent by Hydaelyn.
I think it covers a much broader idea - mortals in general.