Just because a game has the word "unicorn" in it does not mean there are unicorns in it. While my daughter was disappointed by this fact, she and I still enjoyed playing the game anyway. It's another adventure type game with search and find puzzles and interacting with the inhabitants while trying to solve the Mystery of Unicorn Castle.
It is a little more challenging than you might think, so I would say that it works best for a tween than a preschooler or early elementary child. There is also a lot of reading involved, especially when you talk to the inhabitants of Unicorn Castle. The puzzles are generously timed, so it is suitable for young players.
The game itself encourages deductive reasoning, something which few adventure games of this sort provide. Based on that I think it was worth downloading and playing.