It also helped that we all came to similar conclusions about our previous campaigns and why they tended to fall apart, the biggest reason (aside from child disruptions) being that we kind of kept skipping over coming up with backstories for our characters and why they were together in the first place.
Other than one shots, I usually tried to invest in background, to the point if someone wanted some particular perk and came up with a background justification for it, I'd usually go along. Like my most gun-heavy CoC campaign had a Russian mobster as a lead character. He was in America because he had sided with the White movement and when the Bolsheviks took over, he was more or less hunted.
He continued his career of crime in America and maintained international connections, so had ready access to any kinds of firearms legal or illegal. Eventually, though, his crew ended up tangling with an eldritch cult which wiped his group out entirely. His original motivation for going after them was simple revenge, but once he realized what was actually going on, he declared permanent war on the eldritch.
Despite being a criminal, chief among his personality traits were immense patriotism toward his adopted country, absolute hatred of Communists, and ultimately, a willingness to die or face insanity to save the human race.
Cool story bro, you get to have a permanent source of any weapons you want and special services not accessible to normal people.
This wasn't as OP as it may sound because the main threats in that game are not of this world, but it sure provided near immunity to more mundane problems. Also the cops generally looked the other way since the things the party dealt with were things they themselves were terrified of. A few bribes and taking care of problems they couldn't and they'd overlook the occasional protection racket here and there (but when they took "insurance" money from someone it was generally protecting them from something WAY worse than themselves).
The best you can do is work around them when they're that young. You're not going to be able to bring them in as players.
I actually started around nine, with the original Basic D&D box. I was DM since nobody else had even heard of it.
Yes it was total munchkin gaming once people figured it out and the first couple campaigns ended disastrously because of Monty Haul shit. But it was huge fun. It wasn't even known enough that you'd be stigmatized as a nerd for doing it until a few years later.
I appreciate that they wanted to share their hobby with their children, and there's a lot to be said about nurturing creativity, but yeah, they're just not old enough to fully grasp the finer details.
Unless they're interested there's no real reason to force them into it. But if they are, you can pick something for them. I've had this situation and my solution was give them a plain old hack and slash barbarian type character. Or something from some show they liked. If most of their actions are going to be swing a sword, even an interested kid can handle that.
The potential problem is them being disruptive, and of course you have to keep the content friendly.
