After reading some of the other posts, I just wanted to give you a little encouragement. I can't imagine how hard this is for you and your kids. I don't know the legal side of this, but it sounds like you need to able to feel a little more stable in what is going on- you're so thrown out of whack with their dad's whims and moods and changes. I think you'd feel better and make solid decisions if you were more in control (not of him, just of this situation and your own reactions). I would recommend looking on Amazon for some good books about this subject. Read the reviews for them to help in selecting one.
I know you are doing the best you can for your kids, and you are in a horrible situation. Keep trying to do the best for them and keeping yourself sane. Find some kind of support from people who know about bad custody situations.
Don't listen to the negativity, stay positive and focused on what really matters- your beautiful kids. Good luck and God bless!