I truly believe that all children can write! If you don't believe it, just watch any child text message a friend over a cell phone!
Having said that however, writing really has two meanings as it applies in the classroom. There is writing that is the expressing of ideas, using voice, and saying what is in the student's head, (which I feel all students can do), and there is writing that is correct mechanics. Haven't we all had parents say, "My child just cannot write!" when they are really referring to the lack of capitals, punctuation, and spelling.