My husband is a sergeant with the NYPD. In the academy they take firearm safety VERY seriously. The last thing they want to see is a PO's child using a gun and having a horrible injury or death. All of the cops I know are very careful to either not bring their weapon home, or when they do to lock it up very securely. Our gun safe is bolted to the floor in one of our closets and has a combination lock that requires 2 hands to open and a fair amount of coordination. My husband usually leaves his gun at work and once in a while when he's coming straight home from the range or from an assignment outside of his regular area he does bring his gun home and it goes straight into the gun safe.
When my kids were about this child's age (7-8) and they showed tremendous curiosity about my husband's gun (when he came home in uniform one day with his gun direclty from some assignment or funeral) he actually removed the clip and the one last bullet from the gun and allowed the kids to touch it and hold it. He wanted to satisfy their curiosity. but he was VERY clear wit hthem - the only time they were ever allowed to touch that gun or any gun was only if he allowed them to. He showed them that even when the clip is removed there's still a bullet engaged and ready to be discharged. Many people, and certainly all children don't know this. A few months ago, now that the kids are almost 12 and almost 15 my husband took them to a gun range to they could shoot a gun and experience how powerful it is and that it's not a toy. It was a sobering experience for them.
Now all that being said - back to this little boy. he's 7 anything could have triggered that exchange. They might have been watching a movie the night before, or even a TV show with guns. Or he overheard a discussion mom and dad might have had regarding a shooting that wa sin the news, or one that another cop he knows was involved in. Or it could have been as simple as a video game that a neighbor or older brother of a friend was playing.
Alo tof video games are "first person shooter" games taht involve the player in the role of shooting everything that comes along. Call of Duty, Halo, etc. If a child is exposed to this stuff for any period of time it can produce anger, etc. PLus, 7 yr olds don't know how to handle stress, embaressment, etc. When my son was 7 he came home from a friends house talking about shooting everything in sight. This other boy was always playing his dad's M rated games! Obviously, he didn't play at that boys' house ever again. ;o)
Good luck mama. You might ask if her son's been playing at any other kid's house recently. I'd put it down to video games way before i'd assume it had anything to do with dad and his gun - although that's a slim consideration.