Who ils Dawn Allen ? And who is the young girl who spoke with Barry at the beginning of the wedding in the church ?

Dawn Allen is Barry and Iris' daughter. They have twins in the comics (their superhero alias is the Tornado Twins), one of whom is Dawn Allen, the other of whom is Don Allen. Seeing as Jessica Parker Kennedy, the actress who played the Mystery Girl, is biracial and honestly looks like both Grant Gustin and Candice Patton, she is most likely Dawn Allen.