YES your child needs a helmet in a bike trailer if in cycling mode. The "roll cage" that some have described is NOT necessarily a roll cage. In some trailers, it's merely a frame holding the fabric that may not actually contain the child's head. My friend just bought a Schwinn trailer, the frame holds the fabric up but is useless for protecting the child as it sits behind the child's head and if the trailer tipped over, the child's head would smack the pavement. Additionally, even in a 5 point harness a child's head could hit the frame in a rollover.
The 5 point harness is NOT the same as the 5 point harness in a car seat. Think more like stroller 5 point harness. A harness adjuster could slip or slide in a rollover and loosen just enough to allow the child to move enough to hit their head on something.
Also, in the manuals I've read (Burley, Chariot and Trek), the manufacturer's instructions specify to wear a helmet in cycling mode.
Sure the helmet might not help in some high speed crash where a vehicle hits the trailer or drives over it, but a helmet is likely to help in other bike trailer accidents which are parents flipping the trailer when going over a curb or other rough terrain, a collision with a pedestrian, etc.
As for age, the trailer manufacturers say at least a year when attached to a bicycle (can be younger in stroller mode with infant accessories). If a person is going to use a helmet on their child, it's not likely you'll find one that fits an infant. Helmets start to fit at sometime over a year of age. I was actually surprised at how much jostling goes on in a bike trailer, even in one with suspension. And the extra weight of a helmet on those already large heads is quite a bit of load on those little necks.
Originally Posted by vbactivist
Does your child wear a helmet in the car?
Children should be in car seats in a vehicle which are often designed such that child's head will hit the sides of the car seat (often with EPS foam) instead of impacting the hard metal of the vehicle. Vehicles now have side air bags to protect heads from hitting metal and glass. Might not be a helmet, but vehicles are being designed to protect heads in ways they were not before as heads hitting hard objects is cause of serious injury and/or death. Vehicles are also designed specifically to protect the passenger compartment as best as they can and have the least intrusion into the passenger compartment. Vehicles just aren't the same as trailers.