My children's father left us 6 years ago. During the last six years he has been very inconsistent in everything; one moment he is promising us that he is coming home, the next he is caught leaving the other woman's home at dawn. He has made promises to the children that he would be with them for their birthdays or christmas and hasn't kept ONE promise. When the children phone him he seldom answers his cellphone. When they have asked him for pocket money or something he never tries to help them. The phonecalls he does make to the children are not realy conversations, he invariably tells them he hasn't got much phone time, even when they want to tell him all about the bug they found. The children say that he often sounds angry over the phone; it's as if he feels he has to play the part of the good Dad and give himself a noddy badge for the week by phoning the children. The children have seen him an average of once a year and during those visits he is impatient with them, he doesn't take part in activities that would interest them but drags them around to do things that he prefers.
My problem is this; I am being cast as the reason for the children's alienation from their father. It is implied that I am badmouthing the children's father and that the reason the children have no relationship with the father is because I am bitter and divisive. It is true that I have experienced struggled with deep sadness over a marriage that I viewed as a biblical commitment, I loved my husband and miss him even now. As much as I want my boys to have a loving dad in their lives I am not prepared to make excuses for obvious neglect nor CAN I put their father in a good light if he persists in acting inconsiderately. To do that would mean that I negate the children's obvious pain. Children - especially boys seems to want one to put your money where your mouth is and the actions of this father speak louder than anything else.
I can imgaine that trying to build relationships with ones children after all that, must be hard; but if your child is worth it then you would be willing to make every effort yourself and NOT try to make ANY excuses.
Question: How do I avoid being cast as the bitch?

You can't. You will always be the bitch, the bad-mouthing mother who doesn't take care of the kids, because their hair is not cut and you didn't wipe their mouths before a visit. You will never win this fight - at least, not with them.
Within yourself, however, you need to clear this up: if you know you are giving your children the best (in love and discipline) and you know you take care of them, and you know you don't badmouth anybody, you can hear the arguments and accusations and KEEP QUIET. Because you know the truth - as do they, but it is much easier to place the blame on anybody else, than actually doing something about it. You are painted bad, because they have no other way of making themselves feel better.
As for the kids: they know what they see and feel. You don't need to prove anything to them. Trust me, they now who left and who takes care of them. Kids aren't stupid.
BUT: you must NEVER EVER EVER bad mouth the other parent and his or her new partners. As far as the kids are concerned, their dad is the best in the world, and he loves them very much, he is just going through a tough time. (Which lasts very long, sometimes). You are not negating their feelings, you are reinforcing a love they very much need. If you bad mouth (even if what you say is true) you stoop the other parent's level. And the kids will see you see in the same light.

The sooner you accept your fate as being a bitch, the more fun it gets!! Relax, you do your best, let them say what they want to.

The hardest thing you will ever do (even harder than the divorce itself) is not to badmouth him when all you want to do is to lash out and inflict pain (even in his absence) for the pain he has inflicted on you. Dont do it. For the children's sake. And for your own sake. Children have a right to love their Daddy. It is part of their psyche and the damage you do to them by badmouthing him is far greater than the satisfaction that you get from making him look bad. What you do is you make them withdraw and not tell you their feelings when they are angry with him, and eventually you close the lines of communication with your children altogether. It is not worth it. I speak from experience. If he doesnt call or stay in contact, tell them you dont know why, but dont badmouth him. It would actually be a good idea to tell them that their Daddy loves them but is going through some difficulties himself. I promise you if you BUILD the relationship with their father rather than break it down, you will reap the benefits in years to come. They may end up fixing the relationship with their Dad, but if they dont, they will not relate the breakdown of that relationship to you. As for him not doing the same, he is out of your life. Dont let his opinion affect you. After 6 years it is time to move on. Until what your previous partner says and does no longer affects you, you are not emotionally over him. You need to deal with this on your own and not let it affect your children. Get some counselling if you need it (it worked for me) but you do need to move on with your life. Good luck.