Bicycle victoria did a review of these in their magazine a year or so ago. It might be worth checking their website for information, or emailing them to ask.

If I remember rightly, the main danger is not the risk of falling, because basically falls don't happen much (who rides with their kids on the road, etc). The danger is the repeated strain of lots of little vertical impacts as the bike bumps up and down, putting a strain on the kids spine. This is why it is no good for kids under 1, and there is some disagreement, if I remember rightly, about whether it's really safe for a 1yo or not.

So whatever choice you make, I'd be doing some research into the shock-absorption system in the child carrier. Obviously, if it attaches to the bike it will get some shock absorption from the tyres and whatever other features your bike has. I don't know about the trailer-type things, but seem to remember (vague about this), that the shock was an issue with those.

I'm going on what I remember reading a year or more ago. The technology used in the trailers and seats might have changed by now, so I wouldn't totally go on what I'm saying if I were you. But I would certainly be researching the issue of safety not only from the risk of crashing or falling off the bike, but of the safety of normal riding and the stresses it puts on young children.

Enjoy! I'm planning to get a bike seat for DD in a year or two. Looking forward to doing some real cycling again when the time comes.