Yes. A DCFS caseworker or a school social worker can speak with a child at school without the parent's knowledge or permission, and this happens often during an investigation. Schools are a convenient, neutral place for the investigator to talk to a child alone.
School staff, counselors and therapists are mandated reporters in Illinois. If they see an injury or hear something that suggests abuse or neglect, they are required by law to contact the DCFS hotline, even if the family objects.
If your child has already been interviewed, ask the investigator what the allegation is and what was reported. Write down dates, names and what you are told, because that record matters later in the investigation.