Are you talking presently, or in the future? Because I'm not aware of any law that says I would be commiting a crime by refusing vaccination for religious purposes. Do you know the exact RCW?
You are right about children being forcibly taken from you if you refuse medical treatment though. If your child is diagnosed with a terminal illness, and you refuse treatment, the hospital social worker will place your child in foster care and only allow you supervised visits, but no authority in decision-making with regard to treatment. In other words, they can do whatever they want with yoyur child and you have no recourse.