What you believe should happen doesn't change what is currently allowed to happen under the law.

Unfortunately it is very much possible that the parents took steps that were circumvented which would be a valid defense against the type of crime you are referring to. As such it would be a waste of everyone's time to drag that out into court if that is what the evidence showed happened.

If they did in fact have no gun safe or locking mechanism then yes they should be charged but under the laws right now someone using a lock they though was adequate is not likely to be charged and if they were they wouldn't be convicted.