I don't know much about the law, but isn't it the prosecution who has to prove that the parents weren't there, not the parents who have to prove that they were?
In Colorado it appears that if you are a minor and around guns you need a parent present. I know that with fancy webcams it is easy to take a picture of yourself, but it is possible that his parents took the picture. I'm still not sure how a warrant was obtained.
In states that have open carry, are they allowed to arrest you for going into a 7-11 at 11:30 pm with a gun on your hip? Hey, according to the cameras, you might have been going in to rob the store.