So I am going through a nasty, nasty divorce, which hasn't even been filed yet, because originally we were trying to be amicable with each other, but obviously that didn't work out. He has continually threatened me to "take full custody of the children", yet while we have been separated, there have been multiple times where he has refused to watch the children, he has gone M.I.A. he hasn't helped me one bit to provide financially for them. I have good reason to believe that he is simply threatening this out of spite, because he is very bitter about me not wanting to stay married to him.
So my main question would be, should I release the children to him? I am afraid that he will get them, and a. not give them back to me, or b. take off with them to another state where he has family.
He has stolen things from me that he verbally agreed I could keep when he moved out, and when I tried to recover the property from him, he shoved me into my closet doors. I have this on police record. Another time, I came home from visiting my sister with the children, and he was waiting in my driveway, insisted on coming in to tuck the kids in, and the second he shut the door to their bedroom, he accused me of drunk driving and called the police. I asked him to please please leave my home, and he refused, so when the police arrived, they clearly saw that I was fine, and of sound mind, and asked if I wanted to press charges on him for trespassing. Also documented against him.
I love my kids, and only want what is best for them, and it breaks my heart at the thought of them not possibly having a relationship with their father, because they love him, too. Like I said, I am just very afraid that he'll keep the kids from me, and possibly take off. HELP!!

You need to get the divorce filed and ask for temporary custody orders ASAP. However, with what you have documented, I think you can inform him that you will not let him be alone with the children until there are court orders in place.

The next time he asks to see them, tell him this and I would follow it up with a letter to that effect. If you have a lawyer to help with the divorce, then ask them for advice as well.

The issue here is you need to tread lightly. If the court perceives that you are withholding the children out of spite or malice, they will not like it. But if you can show a genuine concern for their safety then you should be OK.