Firstly, let it be known that it is very dangerous to ride without a helmet. Make sure that your baby, and you, are wearing helmets when you ride.
It is only safe to put your baby in a bike trailer when they are over a year old. Any time before is insanely dangerous for your baby. Seat your child firmly, strap them in securely, and close up the compartment.
Usually, the one year rule is a law. This applies in the state of New York, among others. The reasoning behind this is very simple.
http://www.icebike.org/baby-bike-trailer/