Posted on WSDOT's website, under "Washington State Freeway HOV System Common Questions":
"Why do children count as passengers in the HOV lane?
The main law governing HOV lane use is WAC 468-510-010. This law merely states "occupants." HOVs may therefore include passengers who are not licensed drivers. These can include senior citizens, people with disabilities, and children as well as other people who do not, or can not, obtain a drivers license for various reasons.
HOVs with non-licensed passengers do not always help to remove cars from traffic. However, one of the Department’s considerations in determining HOV eligibility policy is the degree to which the policy will be enforceable by the State Patrol. It can already be challenging to accurately determine how many occupants are in a vehicle. It would be much more difficult, and more expensive, to additionally be required to determine occupant age or licensing status. Another consideration is that carpools are sometimes driven by parents or caretakers who transport groups of children to activities. This does keep additional vehicles off the road."
WSDOT - Washington State Freeway HOV System Common Questions