I cannot see how that would do anything but backfire on him and blow up in his face.
Not sure you have a tight handle on the whole "average American's mindset" thing. Threatening a family member or an offspring is bad on its own, but a man going after another man's daughter would be political suicide.
I'm convinced that what the average voter wants to see is the Dem candidate verbally or rhetorically "punch" back, but only Trump himself.
This is campaigning.
All about redirecting and deflecting public attention.
Look, you said yourself you have to hit back against a bully, and how do you do that? By going after the bully's weakness.
Trump's weakness is his daughter.
If Biden made investigating Ivanka one of his campaign themes, I fully believe Trump would not be able to contain himself and would fuck himself over, so Biden didn't really have to do much of anything other than stand there and watch the car crash happen.
Trump already brought family into this by screeching about Hunter Biden.
So I say, let's drag the one thing Trump cares about through the shit and mud.