how do I get custody of my children?

The mother and I are not married. We have four small children, triplets and boy. We have been living together and I have been supporting our famiy. We are both undocumented. I just left her a day ago and am unsure how to proceed. I believe her to be an unfit mother due to her obsessive overeating. She has encouraged poor eating habits that have may lead to health issues in the children. Her constant eating has led her to become overweight which makes it difficult, if not impossible, to physically car for our children. She cannot move fast enough to catch them should one wander away. Her excessive eating is not the only problem. She spends hours a day on the computer playing games and on social networking sites. I dont know what to do about the separation and how to assure that my children are safe and well cared for.