The police officer is correct that she doesn't have to read him his rights until there is a custodial interrogation. Conversely, your son doesn't have to talk to them about anything unless he is detained or arrested and then after his rights have been read.
The store security guard also had a right to question your son as long as he was not being detained. The security guard is only a civilian employee anyway and cannot commit a Miranda violation.
If I were your attorney I would counsel you to count yourself lucky, make sure your kids behave and get on with your life.
In California, juveniles can be held accountable for any criminal offense they commit, just like an adult. However, there are a few twists in juvenile law.
There are some procedural aspects of proving criminal intent (specifically knowing right from wrong and the ability to form criminal intent for a child under 14), Miranda admonitions and when they must be given, how waivers must be sought in order to render a resulting statement admissible, etc.
I can appreciate you wanting to protect your child. Should he face charges and be "in the system," his record does not automatically "seal" when he turns 18, nor does it just drop off his record.
See the link below for some additional information and feel free to call my office if I can help.
You don't have to be formally arrested for Miranda warnings to apply. If the cop is conducting a custodial interrogation, then Miranda is required if they want to use the statements in court.
Custodial just means not free to go; it does not require handcuffs or a formal arrest. Interrogation means questioning a suspect; does not mean aggressive, third--degree grilling. It can all be done very politely. It she's questioning your son about his involvement in a crime while he is detained, it is a custodial interrogation.
She had an obligation to Mirandize him if the State intends to use the statement against him. Also because he is a juvenile, the police are supposed to allow child to consult with parent BEFORE he decides whether or not waive his rights.
The cop called because, having thought about, she realized that she never got a valid waiver. If they tried to bring a criminal action the experienced juvenile lawyer you hire - get a juvenile lawyer - will know how to keep the statement out.
Lastly, I would encourage you to remember that the system can be heavy-handed and though you are undrstandably disappointed in your son and expect better, he does not need a conviction to learn his lesson. Let his lawyer help him benefit from the cop's failure to do their job right. (I say that as a lawyer and a parent.)
When I worked in juvenile many years ago, a judge asked me if I was doing right by a child when I helping him/her escape responsibility (this was after she acquitted my client of assault b/c the DA hadn't proved her case) . I told the Judge - Sure, whenever the court follows the law (or makes others follow the law) and let's a child receive the benefit of a reasonable doubt (or the 4th Amendment), the child has learned about integrity of the system and fair play. The rules are in place for good reason.