Summary: Current child helmets perform well. Research is lacking on the impact tolerance of a child's brain, and some researchers believe that the impact foam may be too stiff. Toddlers may not be able to support the weight of a heavy helmet. If in doubt, take helmet and child to your pediatrician and ask. Nobody recommends taking a child less than one year old on a bicycle.
Although some safety advocates recommend against mounting child seats on a bicycle, several states require a helmet on the child if you do. Helmets for children transported in child seats or trailers should never be the "aero" shape, since the tail hits the back of the seat and forces the child's head downward. You may have to put a cushion behind the child's back to prevent this. If a child dozes in a seat or trailer their head may loll and bounce around, a situation pediatricians say is not good, so either provide a cushion on the sides or some other means to prevent this or be prepared to stop whenever the child gets sleepy.