This is a favotrite of my two tween (11, 12) daughters. I agree with CSM that the world of Zoey is very idealistic, with not many real world, tough problems that today's teens face addressed here. (Perhaps this attracts a wider audience for the network.)
Like other shows marketed for a tween audience, keep in mind that the characters date and have kissing and touching as a problem focus. For example, in one episode a girl (stereotypically, the "nerdy" one) has the problem of never having kissed her boyfriend, and her friends are astonished to the point of pity for her. She spends that episode in search of her boyfriend so that she can reach that milestone. At least, however, once her boyfriend realizes what she's up to, he decides to take control so that the kiss can be meaningful, and not just any old kiss.
Bottom line, wait until your kids are old enough to really start thinking about the opposite sex as attractive, and then talk to them--a lot!