Child seat (or rack) at the seat post I have 2 bikes (mountain and road) sometimes I use them in races (mostly MTB) so I don't want to have any fixed accessory on them. I'm looking for a child seat (or rack where a seat can be mounted) that goes mounted to the seat-post so that I can easily replace the seat and post and mount the seat. I've seen some mechanisms that go into the frame. I thought they could also be mounted in the seat post but I'm afraid it's not a very good idea as the seat and baby weight would be too high and compromise stability and safety when riding. Does anybody have any idea? I'm afraid I'll have to get another bike for that purpose :-P.

I have also though of bike trolleys but they may be too big, expensive and too sluggish.
The closest thing I have seen is the WeeRide Kangaroo that places the child in front of you.  the support bar attaches to the seatpost and to the head tube, so if you had a second seat post you could leave the bar attached to that end and just connect it at the head tube. 


I don't know of anything that attaches just to the seat post, as the physics involved would cause pretty high torsional force at the point of attachment.

See this related question on toddler seat v. trailer for other child carrying options.