RCW 46.61.660 (pasted below) prohibits a driver from allowing anyone to ride in the bed of a pickup. This and other rules of the road can be viewed in Chapter 46.61 of the Revised Code of Washington, also available at the state legislature's website at http://apps.leg.wa.gov/rcw/default.aspx?cite=46.61. In addition to the rule pasted below, letting kids ride in the back of a pickup violates RCW 46.61.688, which prohibits transporting children under 16 who are not properly seatbelted and/or in carseats depending on their age. In addition, all passengers are required by law to wear seatbelts. Youths 16 and over are still required to wear seatbelts and can be cited for failing to do so.
In addition to being illegal, letting kids ride in the back of a pickup puts them at very high risk for injury or death in the event of a collision. See the following links for more information on proper vehicle safety for children passengers, including the high risks associated with riding in pickup beds:
Carrying persons or animals on outside part of vehicle.
It shall be unlawful for any person to transport any living animal on the running board, fenders, hood, or other outside part of any vehicle unless suitable harness, cage or enclosure be provided and so attached as to protect such animal from falling or being thrown therefrom. It shall be unlawful for any person to transport any persons upon the running board, fenders, hood or other outside part of any vehicle, except that this provision shall not apply to authorized emergency vehicles or to solid waste collection vehicles that are engaged in collecting solid waste or recyclables on route at speeds of twenty miles per hour or less.