Wash. Rev. Code § 26.26A.445
(2) If a child has an acknowledged parent, the following rules apply in a proceeding to challenge the acknowledgment of parentage or a denial of parentage brought by an individual, other than the child, who has standing under RCW 26.26A.405 and was not a signatory to the acknowledgment or denial:
[ 2018 c 6 s 510.]