My daughters, 11 and 12 are very into this show right now. They love it. It has some fun story lines-typical problems mermaids would have trying to hide their identities.
However, the characters do not typically share their problems with their parents, and most adults appear to be disconnected, unintelligent, and ignorant. Also, the way the main characters treat their siblings can be very demeaning and disrespectful, which does not exactly help foster peace in a household. Given, not all siblings get along all the time in the real world, but, hey, it would be nice for children to see positive role models, right?
Lastly, most of the characters are dating, and so if you don't feel your kids are ready to be exposed to that yet, you might want to put off their watching this show until they're a little older.
Keep talking to your kids about the messages they get from the role model families they see on television. :-)