Yes. It is likely that your 9-year-old was a witness or a potential victim. Only if your child is a "suspect" does a parent need to be present. So, what the police did was totally proper. They need to separate the witnesses when interviewing them in order to make sure that witnesses are not tainted or influenced by the statements of other witnesses.
The information provided in this answer does not create an attorney-client relationship. If you are interested in his legal services, feel free to call Chris at (303) 409-7635 at his law office in the Denver Tech Center. All initial consultations are free of charge.
They can do that, as both Mr. Leroi and Mr. Luna pointed out. If the child were a suspect and interrogated without the presence of a parent or guardian, the child's statements might not be admissible in court. Additionally some children under ten years old cannot testify as witnesses.
The information provided in this answer does not create an attorney-client relationship. If you are interested in a free consultation with Mr. Bryans, call The Bryans Law Office at (303) 832-2930.