Unfortunately, there is nothing that requires law enforcement to notify parents before questioning a juvenile. They must only notify parents if the juvenile is taken into custody.
Certainly, the manner and methods employed in the questioning may be attacked if it is believed that inculpatory statements were inaccurate or constituted coercion or that the juvenile was unable to understand the nature if the inquiry.
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I don't know the answer to that question. "Is it legal?" is a question that needs to be made more specific. For example, if police question a juvenile at school and the juvenile says "I refuse to answer any questions at all, until my parent and-or lawyer are present," then a Judge may rule the statement "illegal" as involuntary and in violation of Constitutional rights. If you are asking about civil lawsuit for money damages against police, I cannot answer that, and can only recommend you consult a lawyer who regularly accepts police misconduct cases. If so, do so soon since the passage of time will destroy legal rights.
As my colleagues have noted the question is more complex than your question suggests. The question to be asked is, if he is charged, is the statement admissible against him, that is a very different question than the one you asked. You can certainly file complaints, but it may not be misconduct as you suggest. There is not a requirement that a parent be present, but the statement may still be suppressed were your son to be petitioned to the juvenile court.
This response does not create an attorney client relationship and is offered for informational purposes only. Only a lawyer fully versed on the facts and circumstances of your case can properly advise you on the case. I am licensed to practice in Minnesota, not every state. You should always consult with an attorney licensed in your area on how best to proceed.