This is a question that has been asked by many people, and most of the fandom responded with "Violetpaw and Twigpaw of course!" However, how come none look like Pebbleshine or any of her relatives? How come Violetpaw doesn't look like anyone in Pebbleshine or Hawkwing's kin? What if, it's Needletail?
I know I know, it sounds crazy and unlikely. But wait, there's evidence. First of all, Needletail looks like both parents. Her gray fur from Hawkwing and her white spot from Pebbleshine. Not only that, but remember Echosong's vision in the final chapter of Hawkwing's Journey? Echosong said that a cat that looked like Hawkwing was standing over a maple leaf. Not two cats, but one. Why would it be fair if only Twigpaw was in that vision and not Violetpaw?
One thing that many people have said is that Hawkwing's Journey takes place during The Apprentice's Quest. However, we don't know this for fact. Not only that, but remember Thunder And Shadow's bonus scene? Needletail always said she didn't feel like she belonged in Shadowclan, just like Violetpaw, which is why she joined the rogues.
So that's my evidence. I know it might be proven wrong and sounds crazy, but it's just my thoughts!