Small children perched above rear wheels in child seats or towed in trailers do indeed look vulnerable, to the point where we might fear for their safety.
But is there any evidence that they do in any way suffer accidents and injury?
I think not. In all my cycling years and with trade associated knowledge, I've never heard of a single accident involving them. That's probably due to road users taking additional care when they see children in these vulnerable circumstances.
Once again this may be an example of how we have become overly obsessive about child safety and protection.
Children often far prefer alternative transport to boring cars as I well know from the many young kids on the local estates who've begged rides in my large goods trailer on my return trips from recycling. In Holland and Denmark babies and small children are carried in a wide variety of manners on bikes, goods trikes and recumbents, often perched quite precariously or in open goods trailers without restraints, their enjoyment frequently self-evident. Adult safety obsessions all too often deprive children of experiences they are entitled to, and there's little doubt that today's children have impoverished lives compared with those of much older generations.
Life is only worthwhile if it has value, and the wider the experience, the more value a life has.