My Mother Keeps Kidnapping My Children What Should I Do?

My mother lives near my children's School. If my kids choose to go over there after school or something, she will keep them and promise them all sorts of materialistic things and they will willingly stay. I won't get them back for a week or two. They will miss school the entire time and I won't know what's happening. She's done this with my youngest daughter(14) twice and now she is on the second time with my son(17). Except this time she has manipulated him so much he is losing himself and im freaking out. Each time I call the police they say that there's nothing that they can do and that apparently if your child goes willingly to any adult person's house and chooses to stay there, they can legally stay there and there's nothing I can legally do about it unless I have a parenting plan in place or set one up or if i put a protective order on her because then it would umbrella my children and my son would be forced to leave. I find this to be wildly obsurd. So potentially you have to have a parenting plan with literally every single person in your family and friends and strangers? This has got to be wrong... What is the best way to handle this? To file a parenting plan or to file for a protective order? Or can you tell me of a 3rd better option? I'm in the state of Washington. Any help is much appreciated. Edit fixed spelling error