There is no need for anything special. If she usually uses a wand, staff or other focal object, or if you think it would benefit her to try one, then do bring it along. Sirius uses a wand, but I know there are wizards in other countries who do not. We wear robes because that is usual where we are from. If Harri would like some, I can try to obtain some for her, but otherwise she should wear what is comfortable, and will keep her warm, because some spells are better attempted out-of-doors.
I am pleased that you have had such a fine experience at this school! Let me assure you that I do very much want to share my knowledge with Sirius. Although my own education has been disrupted, I have schoolbooks here and I am ensuring that I will at least not be dreadfully behind when I return home, and I am quite capable of teaching Sirius the things that are appropriate for a boy his age. It is good to know that the teachers at your school are magical, and if Sirius were an older boy, I might have considered it, but seven is too young for the expectations and trials of school. He is quite a sensitive child, I doubt he could stand the discipline and such.