My wife and I frequently transport our 4-year-old and 1-year-old by bicycle. They wear helmets and ride in a trailer or bike-mounted seats. People sometimes challenge us, asking if this is safe. The chances of our being hit by a car are low, but the consequences could be catastrophic. Is it O.K. to take the kids by bike when our admittedly safer, albeit not risk-free, car is available? DEREK PELLETIER,PORTLAND, ME.