We recently came across this report from Germany, reported on in a Belgium news site, concluding that a cargo bike or carrier bike is the safest way to transport children. I will admit that my dutch is not much good and that Google translate mostly mangled the article, but from what I can figure, the German automobile club ADAC tested a bike seat, a trailer and a carrier bike. The conclusion was that a trailer was safer than a baby bike seat, but that a carrier bike was safest of all.
Each setup was tested by crashing into an obstacle at a rate of 25 kilometers per hour (15.5 mph). In the crash test the trailer remained upright and the dummy passenger unharmed, while the doll in the baby bike seat had damage to the legs.
The examiner concluded that the cargo bike was the safest, citing the low position to the ground, ability to always confirm that the child is connected to the cycle without looking back, and the larger vehicle size discouraging motorists from attempting to pass as compared to a cyclist with a baby seat.
Of course in America, they still have no clue.