This is a prime example where outside looking in we don't know shit about the situation in full.
The law around guns is very detailed in some ways, not so much in other way, and this may fall in that situation.
https://www.revisor.mn.gov/statutes/?id=609.666 In this case, some of the news states there was no magazine in the gun originally, thus through the law they may not be able to charge anyone as no home storage laws were broken...
Situation sucks as a whole, and I can't imagine the guilt the parent(s) will have on this regardless of criminal charges ever playing a part.