Safety is learned, not instinctive. If you teach your children how to handle something or do something safely and then reinforce it they can learn it and turn it into habit.
Not speeding becomes a habit that is safer than speeding. Not driving while distracted (cell phone, texting, arguing, reaching into the back,...putting on your makeup drinking your Big Gulp of Vodka Sunrise) becomes a habit and is safer than driving distracted. Driving with or without a seatbelt becomes habit that improves your safety. They're not instinctive, but through being taught the hazards of this one activity they can be turned into good habits that lead to safe behavior.
Good habits, not instinct is what helps keep us safe when dealing with any hazard. To develop good habits they have to be taught and reinforced. To be reinforced they have to be consistently followed and when they aren't the failure needs to be discussed, the habit retrained the safe behavior reinforced. Sometimes the failure to follow the habit of behaving safely with certain activities can lead to such catastrophic consequences that the person who hasn't turned that conscious safe behavior into unconscious habit needs to be prohibited from engaging in the activity to keep them and others safe. That usually only is reserved where permanent disabling injury or death is the consequence. That is the case with firearms. Failure to handle them safely can lead to permanent disabling injury or death and failure to exhibit safe behavior should be treated as most serious.
Since your children sound like they are safe with firearms it is probably because you trained them well by making sure they understood the hazards around them and that you then reinforced the safe behavior so they would develop good habits. The fact that they still err and you don't seem to explain the nature of the error means that they are not instinctively safe nor have they developed the unconscious habits to make them as safe as you assume your will has made them.
While I agree that the way to make kids safe with firearms (or knives, or tools, or many things) is to make them familiar with them, to train them in all the ways required to handle them safely, to reinforce that training so that it becomes conscious practice and then unconscious habit, but don't assume that retraining on particular elements isn't needed nor that reinforcement shouldn't be clearly communicated instead of assuming that a raised eyebrow alone communicates enough detail.