As the incidence of it was incredibly small prior to the handgun ban and fathers are still killing their wife and kids without firearms I am struggling to see what impact that ban has had. Or again does it actually matter how someone kills their wife and kids or is the important thing that they kill their wife and kids?
Two points... Read carefully...
1) Do you think, if a father decides to kill his family, they are at greater risk if he is (a) armed with a shoelace, (b) a knife, (c) a gun... In which scenario do you perceive the victims stand a greater chance of survival?
I know you'll have to say no, that it makes no difference. But I suspect you know really, that a gun makes the job so much quicker and easier, is far more effective at killing, it must give the victims so much less of a chance, in more than one way...
And then add to this, without a gun, the individual actually has to get their hands dirty in a far more physical attack, which may either fail, or put them off completely.
In short, a gun makes their odds of survival far lower...
2) I've said this about half a dozen times now. Any stats being quoted from the 90s are not pertinent to this post, unless we agree that infact we're not doing as the OP suggested, where we could feasibly see a large per head count in firearms, instead of the 0.1% being banded around as a valid example.
Now, if we're going to change the goalposts and suggest that infact only a few thousand folks are going to own a weapon, then let's start a whole new thread
And you talk about contrived to me.

So in your scenario we need a kid that wants to shoot up his school. That also happens to have a parent that has a firearm. That also happens to have poor security on said firearms.
Do you really not think there is a connection between children going into their schools with guns to shoot as many people as they can, and how easy their access is to guns? You really can't see a connection there?
The more guns in households, the more likely the two are to bump into each other. And then you're putting your faith in a key, and a lock on a gun cubpoard?
Why risk it? Take up paint ball instead...