Who can legally question a child without a parent or guardian?

Without more specifics being known, there is no blanket answer to this question. Starting with - what is your definition of "being questioned?" Also: WHAT are circumstances? WHERE is this all taking place - on the street - in school - at the police station? WHY is this particular child being singled out if, in fact, they ARE being singled out?

Juveniles are not immune from being spoken to by law enforcement simply because they are minors.

If the juvenile is taken into custody, a parent or guardian or significant adult should be notified and allowed to be with them during the questioning procedure, but juveniles are protected by the same Constitutional safeguards that protect adults.