I am sorry, that is a terrible situation. Try and get a free consultation with a lawyer, because I don't believe you have any legal standing. I think the US Supreme Court ruled grandparents have no automatic legal rights a few years back, I don't remember hearing that that had changed. I have a few links here for you to review, I didn't read them, just a quick google to try and help you.
I hope you are able to work something out, maybe as Amma posted: work with the children's father.
http://www.pbs.org/newshour/extra/fe...ndparents.html
http://www.grandparentsrights.org/