WOW! I feel like I'm replaying an old movie in my head reading what you are going through. I believe your estranged husband is trying to intimidate you. My ex-husband did that for awhile until he realized that it wasn't working.
He used to try to come to the house when he knew my kids were there home alone hoping they would open the door for him; they didn't. Then, he got mad at me because he felt I told the kids not to open the door to him (He thought he was special and the acception to the rule.).
Friends told me not to let him have the kids until we went to court and had an order in place because like one woman here said, there is nothing to say he can't have the kids. He may get them and not bring them back. I've also been told that the parent with the kids at the point you go to court has the upperhand in court. I'm not sure how much this is true, but on some level it does make sense because they don't want to uproot the kids unless the environment isn't safe. I would consult an attorny to find out your rights. If you cannot afford an attorney, I would call around and get a free consultation to find out your rights and get some questions answered. Also, you could check into legal aid where you live to see if you are eligible for that type of assistance (which is based on your household income).
Hold your ground, find out your legal rights, and it will help you to figure out where to go from there. One other thing...DOCUMENT EVERYTHING!
I'm in Missouri too so if you ever need to talk, drop me a line.