If the discharge is coming from the vagina itself then this is unlikely to be a kidney infection. You need to enquire discretely if she has been exploring herself to see if there is any possibility of an infection been introduced. Otherwise it may have been caused by wiping back to front, rather then front to back.
Some girls as they get close to puberty may start to get curious and one possibilty is that she tried to use a tampon, then was not able to remove it. If this is the case then the tampon must be removed owing to risk of toxic shock. However apart from that, she needs to have this swabbed and antibiotics started.