Weeride Kangaroo is very good. I used to have a rear mounted kids seat, but not good on a FS bike. My son loves the position of the weeride, it probably gives him a good sensation of riding the bike – in fact he grabs the handlebars sometimes and does ride the bl**dy bike!
Only issues I have with it are:
You have to ride bow legged, so compromises your riding position slightly. Although I did a 35mile ride with him on last w/e and I was fine.
When they fall asleep, their heads will drop to the side, so you end up having to hold them and riding 1 handed (which I had to do for the last 5miles last w/e!)
Not sure how comfy drop bars would be though!