The answer is almost certainly no. The state-specific law may vary, but generally speaking a child that young cannot be interviewed without an interested adult (who need not necessarily be a parent) present.
The father probably has a strong motion to suppress any statements the nine-year old made because of the nature of the interview. His attorney will know how to challenge it.
"The federal and state constitutions afford no basis for a parent-child privilege, and neither does the weight of common law. As we perceive it, public policy also disfavors creation of such a privilege by judicial fiat." STATE v. MAXON, 110 Wn.2d 564, 756 P.2d 1297 (1988).
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Typically in dv cases their is state/city appointed advocate present for the victim, it should follow that a witness that young should also have a person present during questioning. It is in bad taste to not allow a parent of guardian to be prenset during such questioning. Legal implications make exist depending on the information received at the time of the interview.