Then the Triforce of Courage should be shattered in the Child Timeline too.
Not really, only the physical ToP was shattered, at that point there would be no actual triforce (as an object) on the child timeline.
That still doesn't work. What happens in the Sacred Realm happens in Hyrule, and vice versa. For example, the emptying of a lake will have an effect in both worlds. How does this work with Two Hyrules and one Sacred Realm? You'd have a flood in one Hyrule, so there'd be some sort of equivalent of a flood in the Sacred Realm. But because there's a Flood in the Sacred Realm, guess what happens to the Hyrule on the other side?
But we only see those changes affecting the other realm in ALttP, when Ganondorf has transformed the sacred realm into a twisted reflection of Hyrule. Ando no, you can't say that the sacred realm has always been a reflection of Hyrule, because we have no recent evidence of that and the only other time we get to see the sacred realm is in OoT, where it all looks like some weird "energy place" and not a reflection fo hyrule. I know that we only see the chamber of the sages, but there was no need to actually make it so weird, they could have made a regular temple.
Most of those things about the sacred realm were stated before the timeline was split, so we have no recent evidence of anything regarding the sacred realms laws.
One thing, I haven't played ALttP in a long while, I can't remember any moment in which you change something in Hyrule and that changes sometihng in the dark realm, I always thought it went only the other way. I'm quite possibly wrong, but it would be nice to know.
And you know, what happens in the sacred realm...stays in the sacred realm.
Sorry, had to do that.