I would say it is legal.
RCW 9.41.042
Children—Permissible firearm possession.
RCW 9.41.040(2)(a)(vi) shall not apply to any person under the age of eighteen years who is:
(1) In attendance at a hunter's safety course or a firearms safety course;
(2) Engaging in practice in the use of a firearm or target shooting at an established range authorized by the governing body of the jurisdiction in which such range is located or any other area where the discharge of a firearm is not prohibited;
(3) Engaging in an organized competition involving the use of a firearm, or participating in or practicing for a performance by an organized group that uses firearms as a part of the performance;
(4) Hunting or trapping under a valid license issued to the person under Title 77 RCW;
(5) In an area where the discharge of a firearm is permitted, is not trespassing, and the person either: (a) Is at least fourteen years of age, has been issued a hunter safety certificate, and is using a lawful firearm other than a pistol; or (b) is under the supervision of a parent, guardian, or other adult approved for the purpose by the parent or guardian;
(6) Traveling with any unloaded firearm in the person's possession to or from any activity described in subsection (1), (2), (3), (4), or (5) of this section;
(7) On real property under the control of his or her parent, other relative, or legal guardian and who has the permission of the parent or legal guardian to possess a firearm;
(
At his or her residence and who, with the permission of his or her parent or legal guardian, possesses a firearm for the purpose of exercising the rights specified in RCW 9A.16.020(3); or
(9) Is a member of the armed forces of the United States, national guard, or organized reserves, when on duty.
