What age child should watch High School Musical? How young is too young for a sleepover? Is Harry Potter appropriate for grade school kids? What age should drop off parties start? Ask the next five parents that you see these questions and you'll likely get six different answers to each! As hard as it is for parents to figure out what's age appropriate for their own child, it's even harder to figure it out when hosting a party for multiple children.
If your child wants a theme that you feel is too mature, you have two options. The first is to simply say, "No, that isn't appropriate." The other is to see if there's a compromise. Let's say your sixth grader is caught up in Twilight Mania, even though she's not old enough to see the movies or read the books. Obviously, you aren't going to show the movie at the party but would a life-sized Edward cut out to pose with for photos be ok? What about official merchandise for the plates or cups? Or, a more innocent vampire themed party (a game of pin the bite on the neck, anyone?).
If your concern is less about what you think appropriate and more about what other parents might think then honesty is required. If your having a Harry Potter themed party for grade schoolers and aren't sure all parents will appreciate you showing the movie, then simply be honest about your plans. Before telling parents though, think about what you're willing to do. If enough parents object will you change your plans? Will you offer an alternate activity or will you simply figuring out timing ahead of time so that the guest whose parents object can come later? If your child has chosen a theme or a plan that you're ok with but you fear others will object to, you may need to talk to your child. Explain that others may not be able to attend the party and try and suggest a compromise theme.
If you're hiring entertainment for your party make sure to let the performer know about any age-appropriate concerns you have. If you know your crowd is a little older or younger than typical for the age group, let him know so that he can plan accordingly.
Finally, some parents actually worry the other way. They get concerned that their child has chosen a theme that's a little young. To that I say, don't worry. In all likelihood if your child still likes it, her friends do, too. If not, they'll simply ignore the theme and have a great time anyway!