The number of female characters in works of fiction has increased in the past century, but [[RoundedCharacter rounded]] female characters have been slow to emerge. This is particularly true of children's media, which has few rounded characters of any kind.
The shortage of rounded female characters came about because [[GenderRarityValue the rarity of women in media]] meant that [[SmallReferencePools every female character]] would be a role model for many more children than a male one would. Accordingly, writers and producers were reluctant to portray female characters with ''any flaws whatsoever'' [[ThinkOfTheChildren for fear that they would be encouraging these flaws in girls]].
Children have relatively few problems relating to a FlatCharacter. But as people become older and more self aware they like their characters to have some depth, and this is where this trope can become a little frustrating. Moreover the actual value of a character as a role model is arguably dimished by perfection: real people undergo CharacterDevelopment, but FlatCharacters by definition do not.