WA state child custody laws, procedure for getting full custody of children, other parent doesn't pay child support

You would file the appropriate paperwork with the court having jurisdiction on your case. The other side would have an opportunity to respond. The court would decide based on the best interest of the children.

Generally, the parenting plan and child support are not linked. That is, failure to pay child support is not a basis to deny the nonpaying parent the opportunity to be a parent.

You likely should review your facts and options with an attorney.