You need to stay in where you are and straighten out what's already been going on with your other kids, and tell the truth.
Tell the truth tell the truth tell the truth! You are pregnant again, and that great! A new baby is not a bad thing... try your best to use the resources available to you and get things together for yourself and go from there.
You can totally do it. There are people out there who WANT to help you, no matter what your situation is. Counselors, social workers, mentors, teachers and doctors - they're all there to be used, take advantage of that!
Use the network of people around you... and if you don't have one - make one. If you don't like the one you have.... make a new one.
And don't beat yourself up, or let the other moms on here do it for you. IT is never too late to clean up and get things turned around - and let me tell you, NONE of us have clean hands.
Sending love and prayers you're way - L.