The title of this entry is a common question this time of year. I will first offer three guiding questions and then address the issue specifically.
Whenever a minor you’re in charge of wants to do something that you’re inclined to disallow, I would ask yourself three questions. This thing that your child or teen wants to do:
Is it physically harmful?
Is it psychologically harmful?
Does it unduly tax your resources (e.g., time and money)?
If the answer to all three questions is “no” it is often advisable to allow your child or teen to do that thing, even if it drives you crazy. So often we parents say to our kids “learn to think.” But, what we really mean is “figure out what I think and parrot that back.” Following these guidelines promotes the development of effective decision making skills and discourages dishonesty and excessive dependency.
Let me now turn my attention to the title question. In this instance we’re probably not talking about a risk of physical harm or unduly taxing parental resources, at least most of the time. It’s most likely that we are talking about potential psychological harm. Regarding the latter, a primary question to consider is as follows: Does my daughter generally want to present herself in a sexually alluring manner? Of 
course, there are always exceptions to any general principle, but often girls who typically wish to present themselves in a sexually provocative manner are suffering from significant insecurity about their value in other areas. It’s sort of like (and not necessarily on a conscious level) this: “people won’t find my personality or my skills appealing, so I need to draw them in with my sexuality.” If this is true of a girl she will often attract those inclined to value her primarily for this attribute, and likewise be less appealing to those peers who are operating at a higher level.
But what about a girl who wants to wear a provocative Halloween outfit who doesn’t generally lead with her sexuality, and who will be well monitored during the festivities? For such a girl there may not be much risk of psychological harm in dressing this way on one night. That said, and at the risk of sounding like I’m talking out of both sides of my mouth, it’s been my experience that girls who are secure within themselves often don’t wish to dress in this way to begin with.
Let me try to read your mind regarding two additional questions:
How do I decide if an outfit is sexually provocative?
This brings to mind what Justice Potter Stewart said after he admitted that he could not offer a good definition of pornography: “…but, I know it when I see it.” That said, if you’re a dad you may have a stronger inclination to over-react to even adaptive manifestations of your teen daughter’s femininity (I know my tolerance starts being challenged once one of my daughters’ outfits travels above the knee). So, if you’re a dad it may be a good idea to defer to the determination of a woman with good judgment and a healthy self-esteem (hopefully mom).
What should I do if my daughter is someone who wants to lead with her sexuality?
I would consult with a qualified mental health professional to figure out if her self-esteem is fragile or in need of repair. If it is, there are a number of interventions that can be tried to strengthen her sense of efficacy and value. To find someone by you click here. In closing let me also note that I offer numerous specific and time-efficient strategies for promoting self-esteem, effective decision making and adaptive monitoring in my parenting book Working Parents, Thriving Families: 10 Strategies That Make a Difference. For now I hope you can have fun with your progeny this Hallowee. Afterall, they will all be living away from us soon enough.
Dr. David Palmiter's Blog for Hectic Parents 