A lot of reviews claim that there are some sexual parts which make it inappropriate for children, but the truth is that young children don't see it. This movie is for two age groups: 4-8 or adults, but not because it's sexual.
Although it has good morals, and is a good story, in reality it is very sad. The main character saves her whole species, and gets nothing in return. In fact, she's the only unicorn (the purest creature there is) who feels regret. A lot of people say that it's worth it to her since she's also the only one who can feel love, but she's forced to live every day with the knowledge that she can never be with the one she loves.
In my opinion the moral of the story is quite clear:
In a lot of kids movies, children learn that if you do what's right, you'll be rewarded, which is good for getting kids to do good stuff. But this movie is telling all those kids that it doesn't matter if you get something in return. Sometimes, the good people have to make sacrifices regardless of where it leaves them. I think that making this movie with that moral was a good idea, but at the same time, for kids who are old and mature enough to truly understand the morals, this is a really firm slap on the wrist, and in my opinion is too much at that age.
But overall, it's a good movie, with good morals, and is good for younger kids who love unicorns, and for adults that can handle a little sadness.