Split a large XML file into 3 smaller files, which files will contain the root element of the main file

I have a problem writtin a c# code to split a large XML file into 3 or 4 smaller XML files. The point is not to loose the root element, which means, every single splitted file needs to have the same root element (written) on it and then there are just 8 child elements (depth=1), 7 of them they need to be just written in the splitted files and then the 8th element is repeated many times and contains some other child elements, this is is the element that needs to be "splitted", for example: if there are 28 elements (let's name it ), there should be 9 in one file, 8, 9 .. or whatever in the other files. This is harsh, but I would really appreciate your help. Thank you.