I would feel better about it if it was in a trailer than on a seat higher up on the bike. If we are honest about it one of the few things than happen to bicyclists is we fall down a bit more often than we like to admit. The danger of a fall seems a lot less likely from a trailer where the child is only a few inches higher than the ground. A few wet leaves on a corner and both the cyclist and their child could end up on their side with a child seat on the bike. A little road rash might not be too bad for an adult but I shudder to think of the baby skin getting ground down by pavement. At least that was the reasoning I used when transporting my son when he was small and before he could ride himself.