I am not recommending this (just in case any lawyers are listening) but we started riding with our baby in a trailer at 4 months. We use a Chariot trailer and for the first few months we used the baby sling to provide better support for the small baby - very similar to having the kid in a car seat. Please note that Chariot does not recommend that the baby sling be used when in trailer mode.
Oh well, we must be bad parents. But our daughter LOVES riding in the trailer and we have proven that it is plenty safe. We even managed to roll the trailer by taking a turn too tightly and hitting a curb with a trailer wheel. We turned the trailer back upright and looked inside to make sure the kid was alright - She was still in there napping away (no, not dead or unconscious). I take a wider arc around turns now. Yeah, a helmet _might_ be a good idea, but at this stage she is too small for a helmet to fit, plus I think that she is safer without it due to the higher stresses a helmet would put on her neck. And what can her head hit while she has a 5 point harness holding her inside a big cage?
Audrey is now a year old and have been on all sorts of rides with the trailer ranging from rallies and group rides to easy rolls down bike paths. We would have missed out on a lot of fun as a family if we had followed the official instructions written by the lawsuit-fearing corporate drones.
Again, this is not a recommendation, just relating our experience.