I've never cycled using a trailerbike, but saw a lot of them at the last two Freewheels.
Most of them seemed to use some sort of clamp onto the seat pin, and some even used the childrens own small bikes (with the front wheel and forks removed). I really didn't like the look of most of these, the clamps often looked weak, the geometry of the arrangement often was clearly not good, and in some cases the towed bike was flopped alarmingly sideways, even when stationary, let alone when in motion.
I don't think I'd put any offspring of mine on something so obviously insecure and unreliable. I may have been unlucky, and there are similar designs which are OK, but I think I'd look at something more like the Islabike design.