Problem with my kids visiting their dad and being very mistreated by their cousins? Is legal action possible?

My ex-husband is living with his sister, and my two daughters who are 6 and 4 just got back from staying at their house for the weekend. They told me that while they were there, their 2 cousins who are 12 and 17 forced their heads down to smell a pile of their baby brother's vomit, and they made them stay there with their noses almost touching it. My daughter started projectile vomiting an hour ago and is very sick, probably from inhaling the germs. They also told me they get called names all the time and that the 12 year old tried to force my daughters mouth open to put a booger in it, and that the 17 year old tied a scarf around her neck and was dragging her around by it and choking her. Also, their 10 year old cousin knocked one of my 6 year old's front teeth out playing rough with her, and the 17 year old burned my 4 year old's ear with a curling iron really bad. They also put their s directly on my kids' faces and fart on them. I talked to my girls' dad and he blew it off as just kids playing. I called their aunt, who just denied that her perfect children did anything, without even talking to them. So my question is, what can I do? I can't deny their dad's visitations, can I? I don't want to stop him from seeing them, but I don't want my kids around their cousins unless they get sternly warned that they will get in serious trouble if they treat my daughters like that again. It looks like that's not going to happen though, so what can I do? Is there any kind of order I can add to our custody papers to help? What else can I do?