Constitutional rights for minors is a thorny issue, but generally, children have fewer recognized constitutional protections than adults from a net perspective.
That said, the remedy for a violation of the sixth amendment constitutional right of counsel is exclusion of the evidence gained from the interrogation. Because there is no charge, what remedy would you want?
The comments listed here are of a general nature and should not be understood to create a formal attorney-client relationship -- that is, I am not, and I cannot give you legal advice here. I am licensed in Iowa and to become your attorney, I need to understand the particular facts and circumstances of your legal question and, in all likelihood, need to either meet with you personally or in a fairly detailed conference call.
Is it legal? Possibly. Is anything the minor said after being arrested and not read Miranda rights admissible in court? Probably not. Miranda rights must be read to an arrestee prior to making any testimonial statements made during custodial interrogation (questioning by law enforcement). The remedy is that those statements are not admissible at trial, but may be admissible in pre-trial hearings (probable cause hearings, arraignments, motions, etc). Whether it's "legal" depends on many factors to numerous to mention here.
If the minor has not been charged with a crime, but they may in the future, you may want to talk with an attorney to handle pre-charge representation to attempt to avoid any charges being filed, and thus, avoiding a record being created. If not, have both the child and yourself write down everything that happened now so that you retain your memory of it should charges be filed in the future. The cop wrote a report, you should too. Your other option could be to file a complaint against the officer, although, this could be like poking the bear if charges have not been filed, and lead to the officer pursing that route.
Sounds like a very unpleasant situation. Hope the minor is ok. Good luck.
DISCLAIMER: This message is intended as a general discussion of legal issues and not as a statement of fact, legal advice or a legal opinion. No attorney-client relationship is created by this message. Do not act or rely upon law-related information in this communication without seeking the advice of an attorney licensed to practice in the relevant area.
There is a statute in Washington that requires the police to contact the parents, but only if the minor is under age 12 (I think from memory). It's worth talking to an attorney.