It looks like a prolapse, and the blackish area looks a bit dried out. Keep it cleaned up—a warm shallow soak daily if she is inside where it is warm would help keep her clean, and help the vent area heal. Keep the outside of the prolapse moist with honey, hydrocortisone cream, or vaseline. Make sure that she is pooping. I would not apply antifungal cream to chicks. I think it is up to you as to whether to leave the chicks with her or not. It is always easier to have a broody raise chicks, but it may not be possible if she needs a lot of treatment. You could try it and see how it goes. Make sure that you have a heat source and brooder if the chicks are still tiny, if you have to take them away from her.