I can't answer your questions without more information... before y'all see this as a copout, I would need to know several things... I am just going to take the first two as an example of what I mean.
1) What kind of city street... we talking downtown Manhattan or a suburban neighborhood. Is the kid and his friends riding bikes that fit them, are they wearing helmets... is it daylight, and if so is it during a time when traffic is heavy. Is the kid wearing something dark? I am a cyclist, and I know that there is a HUGE difference in the amount of safety depending on the traffic levels/patterns, time of day, safety equipment, etc.
2) How long has the father owned the motorcycle. Is he somebody who has been riding one for years or did he just buy it last week. The safety level of motorcycles increases greatly as riders become more experienced. Are they wearing helmets? Are they wearing leathers. Are they riding on the interstate, or on a quiet suburban street. What speed are they going? Is it rush hour.
I could go on and on about all of these questions, but you get the point. Many things are much more dangerous depending on how and where they are done, and whether they are using the proper equipment that fits them. There is no blanket "riding a motorcycle" or "riding a bicycle" or 4 wheeler in the woods... people with training and experience can do all three quite safely as long as they don't act like idiots and do something stupid... or they can do something really stupid and get themselves hurt within seconds of starting the activity.
Your sig says "Nothing is foolproof to a sufficiently talented fool" and that pretty well sums up my response to your question... all of these activities can be quite dangerous or quite safe, depending on training, temperament, equipment and training... and without knowing what those variables are, I don't think your question can be answered in any meaningful way.