Quick, read Divorce Poison. I got it last night and have been devouring it. It talks to EXACTLY your type of question ... how to resolve an issue where you think a kid is being encouraged to refuse visits, and is certainly not being DIScouraged from them.
THEN to find out what you should ASPIRE to, check out Mom's House/Dad's house. Another good book with some good ideas on how to wrap your mind around having two separate households.
Finally, when you have taken care of the immediate issue, Google Parental Alienation Syndrome and read up on the things that alienating parents might be doing that could cause or assist in alienation.
It's quite possible that your ex does not INTEND to alienate the kids from you, but does some thing unintentionally that allow it to happen. Things such as encouraging the child to talk about what he doesn't like about you, or maybe simply expressing approval (without words, body language is good enough within families) when he expresses anger or other negative emotions against you. And then there's the ever-present talking to her friends on the phone and THINKING that she's keeping it from the kids... kids eavesdrop... that's a fact of life, and anyone who spews venom at his or her ex to others will find it NEARLY impossible to prevent the kids from overhearing it.
If you work at it, it might be possible to resolve this before the effects are hard to reverse, but be aware that there are SOME exes who are simply not capable of getting it. These are relatively rare situations, but they do happen. Your responsibility is to figure out how YOU are going to react to WHATEVER she does, whether or not you are able to convince her to do the right thign, you figure out how to make the right thing happen from YOUR side, and recognize that you can't make her act right if she doesn't want to (or, in those rare cases, if she is simply incapable of understanding that her point of view is wrong-minded).