Before puberty, girls are prone to vulvovaginitis because their vagina and vulva are thin and less resistant to infection. Bacteria can easily grow and cause infection before puberty, as the vagina is much less acidic than it is after puberty.
Also, young girls are still learning how to toilet themselves. They may not wipe their bottom correctly (you should wipe from front to back) or thoroughly enough. Bacteria from the bottom can irritate the sensitive genital area.