Not likely, but possible. Several of those bosses were manifested, or came to be as they were, as a result of the Adult Link events. Had they never happened, they shouldn't exist, or at least, not in the state we know them in.
Thanks to the time parodox theroy, they would probably cease to exist.
Some of the bosses were created by Ganondorf, like Phantom Ganon, and possibly Morpha and Volvagia. So they would not exist. However, Bongo Bongo and a few other bosses were already alive. Though, it's likely that without Ganondorf's help Bongo Bongo and King Dodongo never escaped there imprisonment. Koume and Kotake (Twinrova) are still alive and return in Majora's Mask, and the Oracle games. So yes, some of them are alive, but no, they probably won't or can;t attack Hyrule.
Yes, they are in child links timeline. A bunch of people use glitches to beat Adult duengeons as Child Link.
Well glitches don't exactly count for anything, and I'd say that King Dodongo is killed by Young Link b/c the paradox happens when the master sword is pulled from the temple of time. Phantom Ganondorf is created by Ganondorf. Volvagia is revived by Ganondorf after the sword is pulled so therefore remains dead. I agree that Morpha is created by Ganondorf. Bongo Bongo is alive (but sealed away). And Koume and Kotake most likely aren't the same in the oracle games or MM (due to the fact that only Skull Kid, Link, and The Mask Salesman are the only people from hyrule that are for sure in Terminia) so they are likely still alive in the Young Link timeline.