I was always under the inpression they had to be with a licenced adult but I found this in the Regs.
WDFW strongly supports and promotes youth fishing. Everyone 15 years
of age and older must have a license. Children 14 years of age and under
do not require a license, but they must actively participate and be able
to demonstrate the ability to handle the gear by themselves. They must
follow all the rules and restrictions for that particular species and/or body of
water, and are allowed the same daily limit as an adult. Adults may assist
and are not required to have a license if they will not be fishing/harvesting
themselves, but children must be present and take part in the entire process.