Also, Brahne and the King (forgot his name, it was said once or twice) had a daughter of her own that looks like Garnet, but she died. And then Garnet came in the boat, so they adpoted her and raised her as their daughter. The only differnece was the fact that Garnet has a horn. And that was taken care of. I just felt like pointing this out.

I do think that at age 6, she'd still have some memories before her royal life. And the brain tends to remember bad times in our life, so she could've remember quite well. But, it can also block these memories, and for the sake of the storyline, that's what it did.