How can I help my granddaughter get over not playing with the neighbors kids, they are bullies?

Last week we had a falling out with our neighbor's. They moved here from Las Vegas about 2 years ago, they have two little boy's, one 6 yrs and one 4 yrs. These two little guys are very mean and disrespectful. They have repeatedly put their hands on my granddaughter age 6 yrs and my grandson age 3 yrs. The hitting has been the biggest issue for us. These two little boys are not very nice to each other either. Last summer the older boy pushed my granddaughter to the ground and choked her so hard that she almost ped out, she cam to me and told me about this, the father happened to be outside talking to us, he made the boy go into the house, while the mother acted like it was my granddaughters FAULT! We have had many problems with them hitting. Last week they were again playing outside, the youngest boy took his fist and planted it in my granddaughters face, eye. Again, we were outside visiting with the parents when i noticed my GD was not outside, I went to find her, she was upstairs with a cold washcloth over her eye crying, I asked her why she did not come tell us, she replied I didnt want to cause a problem with everyone. I was so mad, I reacted by going ooutside with her in my arms, and called the boys to "their" driveway, to ask if the boy hit her, why sis he hit her? The oldest boy ran to his mom and said he didnt hit her, while the yongest boy said he did hhit her. I suggested that we let my GD tell her side of the story, the mother started yelling at the top of her lungs at me telling me I was a trouble maker. It ended up with her husband calling me a name, them ttelling us to get out of their yard. I have decided to just not let my Grandchildren have anything to do with them.