What can I do to make sure that I obtain custody of my children?

I have two children, their father and I were married, but we separated in 2007 and have been ever since. We still have yet to file for an actual divorce. He recently attended a legal aid class to file the necessary paperwork to proceed with a divorce but we are still legally married. Since we've separated and even when we were married he was never apart of the children's lives, didn't attend birthday parties, Christmas', Easter, etc. And only provided for my 8 year old financially sporadically and has provided nothing for my 3 1/2 yr old since the day she was born, my fiance does and has since she was 5 months old, with the exception of the court ordered child support that he is made to pay for the past 2 1/2yrs. In the past 9 month he has come to pick them up 1 time to spend a Saturday from 5:00 pm to Sunday till around 2:00pm. He lives within walking distance to us& on numerous occasions I would offer to bring them to him, pick them up from him&even provide food for them so there was no excuse for him not to be able to see his children other than he simply doesn't want to, he constantly makes promises to them and then breaks them and I want to know what is my best legal option as far as me obtaining 100% sole custody of my children to protect me&them from his unpredictable ways and how do I go about starting the process?