you have to be with them, and even then, imo you should keep to clear and short routes. Traffic here is dangerous, no doubt about it.
People here are aware that roads are dangerous so MOST will drive real slow, of course the percentage of dipshits who make the road dangerous is still too high. I have been hit twice by a car. when i bike with my boy (3) he is on my bike and I bike very slooooowly.
BUT, there is some good news. Once you get out of the city (Third Ring Road) the biking is great. Wenjiang Green Belt, another couple of green belts south side. Biking out there is great for kids. In the city, I would check it out yourself before you put them out there ….