I think Princess Cadence could have been a royal fillysitter. It didn't show the Princess getting any bits or anything like that, however. Also, Twilight Sparkle was Princess Celestia's personal student thing, AND she was Shining Armor's little sister, so she was involved with royalty either way. Princess Cadence was always spreading love and such, she was loving Twilight Sparkle, in a way, as a mother/daughter, perhaps?
Princess Cadence could be a royal babysitter or an arbiter as others have said, but I'm honestly not sure. I think that it would be great if she appeared in a few episodes in season three, and more of her past, as well as her royal duties, are explained.
In my opinion, I doubt that she's the royal foalsitter. It seems like a job too belittling for royalty. I just think that she spreads love only.
She's the princess of love and the ruler of The Crystal Empire and the third best princess, Cadence rules.
