My fiancee's ex hit me and now shes keeping his children from him... do I stay or go?? help!

Ok I've been in a relationship with the love of my life for a yr and a half now, he has 2 children from a previous relationship whom I adore to death even though they get stressful hehe...

Anyways the ex has never liked me and always caused problems in our relationship. I used to be allowed around her place to pick up the kids until one day she just snapped for no reason and said I wasn't allowed near her house. Out of respect for I haven't been around her house since that day, the acception to the rule is though that she's allowed at my house to drop and pick the boys up also. I wasn't happy about this but I kept my mouth shut to keep the peace. The other night she went of at my partner for not paying his child support because he hasnt been working atm cos he just got fired. And I'm in the belief of he has no money to pay and they are my responsibility so I wont pay. Anyways he couldnt pay it this week so she went off we had the boys that night she rang my partner said

" I'm coming to pick them up now" and started mouthing off because she didnt get any money. She comes around my house and she gets out the car and shes yelling and screaming, I'm standing in the hallway watching tv and she can see me so she starts calling me names and all sorts of things and I've been dealing with this for a long time now. But it got to the point the other night where I had, had enough. She called me " A fat B***h and you better stay in that F****n house." So I went outside and said " Get the f*** off my property" cos at this point I'm so sick of it all I do the right thing by her to keep her happy and shes a psycho. Anyways she comes running from the car and I walk down on to the grass and she mouthing of at me and i just keep telling her to get off my property so then she pushes me and natural instinct is to push back. Then she starts hitting me in the head... Mind you doing this infront of the kids.. I didn't hit her back I just kept trying to push her off me cos there was no way I was gonna fight infront of those children. She mouths off for like another 10mins and then gets in her car and drives off. All of this conflict has come from my partners cousin because she tells the ex things that arent true then runs to my fiancee and says things to the ex. She seems to be playing everyone against each other.

Now she's told my partner her ex that the only way he can see his children is to go to his place because I'm a violent person. I'm really upset over the whole thing cos I have done nothing but try and keep peace and I did nothing but tell her to get off my property.

This whole thing is hard to deal with considering the fact that it could cost me my relationship because I would rather walk away than have him be kept from his children. And because now she's going around telling people I hit her first when I never did I only kept pushing her away. So its messing with my head..

Has anyone else been in a similar situation and how has it gone? Any suggestions as to what I can do or what my partner can do?

I'm in a simliar position without the violence. I don't know whether i will continue my pathetic excuse for a relationship, i just hate it. But, when times were better, the person i am dating and i were trying to get a parenting order to enforce rights to see the children. You might want to consider this.