Give subcategories a different CSS style

Is there a way of separating the child categories from the parents, so I can assign a different CSS style to the children?

So, e.g. Have the posts from the Parent category on the top, and the children’s posts below in a different style, with the heading above each category.

I want them all to be on the same page and I want it to be a generic thing, so when you get to the lowest level in the hierarchy, the posts will appear at the top of the page.

Thanks! 🙂