I cringe anytime I see children in a seat on a pedal bike. I'd wait until your first accident before you put a kid on your bike. It might be 500, 1000 miles before you crash, could be something as simple as grabbing a handful of front brake on dirt (my buddy destroyed his shoulder going 30... I saw the whole thing... Didn't look too bad till we saw the X-rays) or something s bit faster like a lowside on a small patch of wet asphalt going 35 (wrist) my first "slip" was braking at 10mph front brake only on dirt. Not even that much braking. Just a tiny bit and the front slid right out and got all sorts of bloody scratches on my unprotected flip flopped feet. Been riding with shoes and pants since.
As a "total newb" I would not be worried about the noise so much as the integrity of the bike. Wheel bearings, rear rag joint sprocket, fenders, tires can all stop you in an instant if not done right. Wear at least a 3/4 for up to 35mph and a full face above that.
BTW if you still wanna, instead build a sidecar attachment for your kid. You can really build it up with a six inch crumple zone on all sides, and a sort of safety cell for your kid with a small roll cage. Cars will take you more seriously too.
