I don't think the full story will change anything for me in this, a driver could not be in full control of the vehicle with a kid on his lap, vision obscured, kids arms and legs flapping about.
It shouldn't happen full stop, quiet road, busy road no matter it's just crazy.
I had both my kids behind the wheel by the time they were 7.
Neither of them could reach the pedals, so the stop & go motion was down to me while they were on my lap.
Their arms & legs weren't "flapping" about in the slightest, and as I was in control of the stopping & starting....we cruised along at a walking pace while they mastered distance & perception.
I'm sure you'll have a PC answer as to why that was dangerous for anyone else to be around us while that was taking place, but tell me what was really dangerous about it.
Don't get me wrong....if Martin's neighbour is hooning (hooning = more than walking pace) around an estate and letting a kid have control during the process...then someone needs to have a word.
However if it's just a case of dad letting the kid steer at very low (and controllable by the father) speeds on a nice quiet road, then I don't see the problem.