Yikes! Sounds like a horrible situation! You say a "parenting plan has been started", but it's not clear if there are orders in place. If there are orders in place, and she violates those orders, she can be pursued for contempt by the father. Even if there's no order in place, it's probably still not a good idea to simply disappear. The State has a confidential address program whereby DV victims are given a PO Box in Olympia to use to get mail and for some service purposes. She should probably talk to her social worker about getting one of those and changing her address with the court, and maybe getting a proxy address for service. Obviously, her safety and the safety of the children are paramount, but there are ways of going about this to protect her safety while protecting her from contempt actions.
If it truly is a parenting plan, a Notice of Intent to relocate would be required. Likely the other parent will not cause a problem. If the law is not followed, a complaint of contempt or custodial interference is possible. Although that is an issue, the biggest issue is safety. In this day and age I can almost always find someone. Huge steps need to be taken to protect yourself. Ms. Andrews is right on about the confidentiality program. However, not seeing the child for a few months or a few years does not terminate parental rights. The father may very well be freaked out when he finds out about the situation. Seattle has some great domestic violence resources. Not the least of which is through the YWCA. The confidentiality program info is at: http://www.sos.wa.gov/acp/