I think that is true for very young children; my son is less than three, is just developing a concept of gender, and regularly tries to take his clothes off whenever he feels like it. I'd be more concerned with the pre-pubescent ages, around 7 to 10. Studies show that kids around this age, who get exposed to more sexualized content, develop more negative associations, like being more likely to associate a person's worth and their sexual appeal, more likely to sexual objectify themselves or others, and being unable view human beauty out side of sexual attractiveness.

That being said, seeing one girl in a risqué outfit is more likely to lead to an awkward conversation for a parent than anything else, but in my opinion, its probably not healthy for this kind of cosplay to be the norm at comicon if comicon is going to remain an all-ages event. But again, this line is blurry, and it's possible if just become an old prude since having a kid.