Different strokes,
Personally, I would never put my child on the back of a bike. In all honesty, I would never put anyone on the back of a bike. Motorcycling, simply, is not safe enough to expose a kids body to, IMHO.
Now, I dont think someone is a bad parent if they do, they're sharing with their kids... But it is very, very dangerous. A small crash could be life changing.
I have had my daughter riding sidecars with me since she was four, though...