Post-Miranda questioning likely refers to a situation where a person is questioned by police after a Miranda warning about their right to be silent, consult an attorney first. Though I am unaware of an appellate case or statute which requires police to inform juveniles of their right to have parent present during questioning, a juvenile does have that right in Minnesota. If they are lucky, the juvenile will refuse to answer questions without a parent being present (and a lawyer). If so, police would be required to stop. Usually, they will then seek out a parent and try to persuade the parent to allow it.
Questioning a person "Post Miranda" means that after a Miranda warning was given, and consent to question was established, authorities questioned a person they were detaining. As for questioning him not in your presence, the laws in various states differ on that depending on the state constitution. The failure to notify the parents for 7 hours may in fact render the statement coerced. Further the government will have to prove your son made a knowing waiver of his rights.
Anytime a person is charged with a crime, even juvenile, a lawyer should be involved. There are numerous rights and opportunities that the public is unaware of that need to be asserted. Hence you ought to get an attorney involved in this case as soon as possible.
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The answer involves both State and Federal law. Custodial Interrogation requires Miranda-type warnings, and if one asks for an attorney, interrogation is supposed to end. what is custody and interrogation for purposes of miranda "kicking in" can vary with the facts of an episode. the role of parent in this analysis may well be important, or not, depending on, again, the facts. a youth asking to talk to a parent who is also a lawyer might force questioning to end. some states do make parental involvement, even permission, a requirement for juvenile questioning. many do not. my own state(Oregon) does not hinder law enforcement interrogation by adding a "parental notice/permission" feature to the drill they need to do with a juvenile suspect. asking to speak to their parent, or a parent trying to invoke miranda rights from afar, do NOT make further questioning unconstitutional without more, much more perhaps. a parent's presence may, and may not, improve the law enforcement officer's ability to examine a youth. it is, in many quarters, left to the officer to decide what role a parent is to have during their child's questioning. the maturity of the child, the conditions of questioning, including time of day/night, fatigue, style of inquiry...these facts inform the voluntariness of a youth's statement. voluntariness is every bit as important to the reliability and admissibility as whether miranda rights, which are intended to preserve one's right not to incriminate oneself. a good juvenile justice lawyer in your jurisdiction is needed here to adequately analyze what was done with your child.