I lived with a women for 16 years, do I have to get a Divorce.?

I lived with a woman for 16 years and we have four children, we live in Iowa and are now separated, I'm wondering if I have to file for a Divorce before I file my 2010 tax return. We filed jointly as married for those 16 years that we were together. How will the kids be decided on the tax forms can we split the the kids two each. Do we need some kind of legal Doentation.