In the majority of cases you maybe correct. But there are 12yr olds out there that don’t share your motives for building an explosive.
Personal responsibility is great when it works with proper parenting. You’re going to have to deal with the real world when schools around America start blowing up to match the ideal.
Maybe as a society we then will have to have the conversations about cause/effect and personal responsibility that prepare children for adulthood.
When I was 9 my brother gave me a disc with all sorts of hacking and explosive making material. Jolly Rodger cookbook, handbooks on counterintel, hacking books about overflows.
I had great fun making thermite and smoke bombs and it led me to my career in software.
None of it was any more dangerous than climbing a tree because of good parenting.