Negligent, willful mishandling, storage, making a attractive nuisance of any item which then is used to cause harm, should be, likely already is, against the law in most United States civil locations, towns, cities, perhaps States.
Enforcement is what is lacking when such behavior by adults is overlooked, allowed to pass unaddressed by local Law Enforcement, as well as Landlords where such cretins reside, either as a legal parent, or as an invited guest.
The 11 year old can be educated.. perhaps by being reminded that the weight of near any loaded firearm is considerable greater than any toy as not to be "confused, pointed, and the trigger pulled"... Perhaps the 11 year old, his 5 year old brother, and any other children in residence need to be removed by CPS, at least until the "parents" and other adult beings responsible finish their jail service time.
There is no 'Good Guy' involved with such an incident. The inanimate object which was negligently or willfully made available, needs to be properly secured, as it should always be, and, if needed, confiscated by local Law Enforcement until such time as the criminal negligence or more serious charges, and disposition of vulnerable juveniles, has been fully adjudicated.
A loaded, or unloaded, locked firearm, legally secured is not a threat, any more than a similarly weighted brick or other inanimate object may be thought a threat to anyone. Law Enforcement should 'run' the adults associated with this residence, and these children, for additional info, such as Parole Status, Immigration Status, and Warrants as well, as the behavior 'warrants' a check for other illegal involvement.
Guns do not kill, harm, or wound people; people harm people.