thinking about the art-verse as i continue to prep for the sequel and like -- one of the things that is super fun about it is that jenny's decision to not tell giles was a supreme act of love that utterly shattered her in every way possible. can you imagine a world where giles KNEW he had a kid with jenny living in another state and was not allowed or able to see that baby? the man is intellectual and thoughtful -- as much as he might initially push for the kid to be raised on a hellmouth, i do think he would see jenny's point in relocating art to a place where she knew he would be safe from the supernatural weirdness his life is tangled up in.
and that would just in turn be food for giles's Complex about how weird his life is, and he probably would have spent seasons three through seven in a self-loathing tailspin for not wanting to be a watcher anymore, because how could he POSSIBLY not want to be a watcher when buffy NEEDS him to be one -- he would have been wracked with misery and guilt. and it's not impossible that he wouldn't have told the kids about art, which means he's keeping a secret that he feels horrible about! literally living a double life! it would have been marginally easier on jenny but SO much harder on giles. she made the choice to leave for art's benefit but the choice to not tell giles was because, at the point in time when she made that choice, it seemed legitimately impossible that they could ever be together, barring, like, buffy dying (which is obviously not something that jenny would ever want or hope for).
so telling giles would just mean burdening giles with missing art for however long he was tethered to the council, which jenny assumed would be forever. whereas not telling giles would mean that giles could (theoretically) get over that lady he dated for a year who left him. every second jenny held her tongue and didn't tell him was for his benefit. eight years and she loved him so much.