The most common question I get about biking with my daughter is whether or not it is safe. I've also noticed that on any thread on any forum or list that addresses biking with kids, someone says some version of "You are all really awful parents. You should never bike with children in X setup or before/after Y age. Keep them in a car where they are safe."
My short answer is that, no, I don't think biking is safe, for adults or kids. Cars are big. Bikes and people are small. People are soft. There is very little space for bikes on American roads. But driving in cars miles and miles a week isn't safe either. Commuting on the highway every day 10-30 miles each way is unsafe, but it is a risk that we as a culture consider normal.
But I do think that biking is safe enough, and that on balance, once you consider the other alterations we've made to accommodate car-free living, our safety risk (for both grown-ups and kid) is not dramatically worse than the average suburban family, in which members drive (or are driven) to virtually all work, school and activities.
This "safety assessment" on my part has been based primarily on 12 years of experience with big city biking, and my own gut feeling, which is perhaps not the most scientific method. But recently, on one of those threads where folks got bent out of shape about safety, someone pointed to a quick analysis that actually seems useful. In 2005, bicyclist deaths were only about 2% of all traffic fatalities in the US. However, that doesn't take into consideration how few people bike, or how far they bike. If you also take into consideration the fact that only 0.2% of miles traveled are by bike, then you can get a ballpark estimate of actual risk. Assuming that all remaining miles and all remaining fatalities were in cars (which isn't perfect), biking results in 10 times as many deaths per passenger mile as driving. In MA, the situation may be a little better, since the 2006 stats put bike deaths at 1.4% of traffic deaths, but I couldn't find any MA specific data on miles traveled.
Now, 10 times as risky as driving sounds pretty bad. But this is per passenger mile, and people drive much longer distances than they bike. Our daughter is almost never in a car, except when we travel (this weekend's trip to visit Woods Hole for instance). In our daughter's case, she's on the bike less than 5 miles per week. Thus, even with this overestimate, her accident risk due to biking is less than any kid who's in a car more than 50 miles per week, which I'm guessing is a lot of kids in car-dependent families, particularly those who travel long distances to school.
The situation might be a bit different for me, since I commute the longest in our family, at about 5 miles total per working day. Thus, someone would have to be commuting at least 50 miles per day for me to beat them out safety wise (for commute only). But here, I'll note that we almost never drive for those extra errands, to the store, or to visit family, so it may well even out once you consider non-commute driving, and that doesn't take into account health benefits of exercising instead of just sitting in a car, or the fact that safe traffic biking practices and basic safety considerations (like lights and helmets) reduce deaths.
Similar analysis for pedestrian stats is also somewhat sobering, resulting in 15.7 times the driving risk per mile. But again, people just don't walk very far so similar logic holds. Interestingly, no one ever suggests that you shouldn't walk somewhere because it is "unsafe."