This is a phenomenon I see all the time when I’m shopping (which is a lot). I try not to get too exasperated about grammar mistakes — the fact that they’re so common keeps me in business — but I honestly don’t understand why this is such a common error. “Girl” doesn’t end in “s,” or a vowel, two things that can trip people up when it comes to employing the possessive. This is a simple rule: If something belongs to a collective, you use the plural form, followed by an apostrophe (girls’).
Oddly enough, all the other signs in the store were correct, including “Women’s” and “Men’s” which seem to have more potential for confusion. I don’t get it.