Is it against the law for an investigator to question a minor without the parents knowledge?

You don't give much by way of the specific fasts. For example, was the investigator a police investigator; where did the questionng take place; and other like matters. That having been said, the general rule is that, the police can question a minor without their parents present if that minor is not in custody. Therefore, when an officer has a reasonable belief that a minor has violated the law they can detain the minor to conduct an investigation. During this initial detention, the police are not required to let a minor call their parents.

However, once the minor is in custody (i.e. under arrest), then they have the right to call their parents and have their parents present during any questioning. Bottom line, anytime that a minor's Miranda rights are implicated they have a right to speak with their parents and have them present.

Note: Regardless of age, if someone is in custody and not given the Miranda warning, then is such case no statements made by them can later be used against them. However, prior to being placed in custody any statements made can be used. If they were read their rights and subsequently made voluntary statements, then those statements may be used as evidence against them.