Of course situational, and laxly enforced in any case. The o.p. could ride on the sidewalk for years without being cited if they rode in a courteous manner. That said, "dangerous" is a perception. I know very fit and able bodied persons who think bike lanes are instant death and do not use them, ever. They ride full size bikes on the sidewalk with perfectly serviceable bike lanes right next to them. OTOH in many areas of my city (and I am in downtown!) don't have sidewalks. Pedestrians, skateboarders, people with motorized wheelchairs, all have to, and do, use the bike lanes because the grass verge that stands in for sidewalk in these areas is lumpy and full of holes, debris, etc. FWIW.
There are 16" bikes, 20" bikes but 12" sounds like a juvenile size and $400?/30lbs.?? Does not compute. I think we can scratch the custom Bike Friday. A Dahon Boardwalk? 20" I think but perfectly reasonable for sidewalk use. A Razor scooter as someone above suggested would be ideal if one really needed something a lot smaller than 16" wheels with reasonable build quality.