Anyone can post bail for anyone. In this case since it is for a juvenile the parent or legal guardian must be involved for the kid to be bailed out and let out of detention. So if the parent isn't willing to allow the kid out then an aunt or grandmother can't just go and bail the kid out.
I agree with the answers already posted, however, it will also depend on the specific juvenile facility, not all of the juvenile facilities in Washington allow juveniles to post bond to be released.
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