Kids are very smart, and tend to be a lot less "complicated" than adults.
I would just give a "kid version" of the same thing I would give an adult.
For example, instead of "2nd Amendment says I can", I might say, "because sometimes good people have to defend themselves."
For kids this might work, "Because adults have a responsibility to protect kids as well as themselves."
Just keep it a level that does not need 12 years of mis-education to understand. Keep the politics out of it. Just keep it really down to earth. Kids will get it.