Do you have a child support order in effect? What about a divorce decree? There are father's rights, you can ask for a hearing in the domestic relations section in your county and ask about a set visitation schedule and every other holiday. I know here in our county to request such a thing it's only $10 and you get a mediator. They will also want to speak to the children if they are old enough to express their own opinions (they will speak to them in private without a parent present).
I am really really sorry you are going through this, please dont dispair, help is on the way, one day she is going to need something from you and DH and all you have to do is say NO! She's doing all this because your DH doesnt fight for his rights, believe me I know, my DH never did and then we went to a mediator, who settled everything.
When you do go you have to tell them everything, my DH told the lady that he was tired of the mother bad mouthing him, talking about his wife (me), etc... it was mentioned in the ammended child support stuff that:
~ At no time should one parent disrespect the oher parent by any means.
~ No using foul language on front of the children
~ No drinking or smoking in front of the children
I am a smoker, she had to complain about the smoke (but I never DID smoke around them... EVER!) but it didnt bother me, she even fought about NOT having them stay over a night, changing visitation times to her convenience, etc... it all backfired on her because all we had to do was talk in front of a mediator and let it all out, the mediator saw right through DH's ex and gave us everything we wanted.
Good luck AND let us know how things turn out, I am praying for you and your family.