It is implied that Shelob may have fled from Ered Gorgoroth as Tolkien states, "There agelong she had dwelt, an evil thing in spider-form, even such as once of old had lived in the Land of the Elves in the West that is now under the Sea, such as Beren fought in the Mountains of Terror in Doriath, and so came to Lúthien upon the green sward amid the hemlocks in the moonlight long ago. How Shelob came there, flying from ruin, no tale tells, for out of the Dark Years few tales have come."
(Gorgoroth was Ungoliant's temporary home in Beleriand)
Most online Tolkien references take "child" literally as well, listing Shelob as Ungoliant's daughter, so it's a pretty common perception in any case.
The only reason I think there might be any generations in between at all is that Shelob is clearly greatly diminished from Ungoliant. Going from "can eat the light of the trees and give no fucks." to being horribly burned by a thrice-removed version of that same light in one generation is quite the power level drop off, and is sort of out of line with the way Tolkien treated that sort of thing. Unless maybe Shelob was the "runt of the litter" or something.
I think part of what leaves room for ambiguity is the fact that Tolkien variously decided at different times that Ungoliant was a maiar who was originally a servant of Melkor, some spirit of the outer dark beholden to no deity of creation, or something vaguely roughly in between those two power levels (which is roughly what was published). It's clear she's not more powerful than Melkor until she consumes the light from the trees, but powerful enough for Melkor to pretend to treat with her on equal terms before that.
Incidentally, I tripped over a fun fact while double-checking stuff:
The name Shelob is derived from "lob", an archaic English word for spider. A variation, "cob", is the derivation of the word "cobweb". The first element, "she", simply mentions the spider's gender.
