Given how accurate toys today look, and that it's a 9-year old, I'd think most people would assume it's a toy gun and move along.
Quite the opposite - law enforcement officers are trained to treat any gun-looking object as if it were the real deal.
And a real, operational firearm in the hands of a nine-year-old is very much a cause for concern. Nine-year-olds don't really have the mental faculties to comprehend the risks associated to firearms and should never handle them in any case*. So even if the gun itself was secured and not in operating condition, the response to it could easily have been a lethal one.
Worst case scenario (in this case): some people get scared. I also doubt a 9-year old kid is threatening enough to cause a stampede.
Worst case scenario in this case is that he would have been playing with the gun in public, someone called a police who didn't know how to deal with a kid holding a firearm, tried the standard approach of pointing gun at the person holding firearm, telling them to put the weapon down and all that jazz, scaring the child, he points the gun at the police officer, they fire at him as is pretty much the SOP when police officers are threatened with a firearm.
The severity of the reaction here was not inappropriate, but its target was. Nine-year-old cannot be held culpable of these kind of events, whereas grown-ups should be responsible for the lethal and lethal-looking stuff they own for whatever reason.
*I'm talking about actual firearms here. Young children have no place operating cars or heavy machinery because they don't understand the risks associated with them, they don't have the situational awareness and attention span of a normally developed adult. Similarly, they should have no access to lethal weaponry such as firearms. It really boggles my mind how the US population have no problem with requiring a driver's license, while having this insane aversion against some mandatory training or age requirements for owning a firearm.
If we're talking about a family hobby, there are other devices that are more suitable to children, which can be used to familiarize them with the concepts and safety rules of firearms. BB guns (both air pistols and longer types), airsoft/paintball, and archery are all much safer than actual firearms.