What are the grandparents rights in Washington state?

We live in Washington and my parents live in Nevada. I recently had a falling out with my father and will not speak to him ever again. My children are still pretty young and really only talk to my dad on the phone every once in a while but I know my father and if I told him I didn't want him to see the kids that he would try and find every possible way to garauntee some kind of visitation with them. He is sneaky and manipulative and is one of those people who will tell the kids all sorts of lies to turn them against me; he's a brain washer. I moved out my senior year of high school just to get away from him (in fact, out of all 4 daughters 3 moved away before the end of high school, the 4th tried but couldn't follow grandma's rules so she had to move back to our parents house...no one likes the man or can stand him...my mom barely tolerates him and only because he's brain washed her into thinking she can't make it her own even though she is the 'bread-winner" in the house, he doesn't work, hasn't since I was in early grade school and all this is why I do not want my children around him). Is there any way to keep him from seeing them if he were to sue for rights or whatever?