I haven't been following the case. If it was a gift, then it can't be a straw purchase. I would have expected the basis for criminal negligence to be that
a) The parents knew the child was "troubled" and threatening violence.
b) Any reasonable person would realize the need to secure the firearm so that it could not be used without supervision.
c) They failed to secure the firearm.
I vaguely recall something about a conference that day, and perhaps knowing that they had failed to secure the firearm, a reasonable person would have been terrified on that day that the kid was going to shoot up the school if he had managed to gain access to the weapon. At any rate, they, having the knowledge of exactly how secure the gun was or wasn't and the means of determining whether it was still in the house, might have squandered the last clear opportunity to prevent it.
I still wonder whether deep down the mother wanted it to happen and just couldn't herself foresee that she would be prosecuted, maybe a munchausen by proxy thing, or worse.