how young do people on this forum take their children on bikes?

I am buying an xtracycle (see my other post about the radish) to break the car habit, and the other day I picked up a second-hand child seat, but the instruction manual says 'not suitable for children under one year old'. My baby is only 4 months- surely i dont have to be a slave to the car for the next 8 months? (and then again when we have another...).

What do other people do about this problem - do you manage to carry babies on a bike, and if so, how?

Also, what's the view of child helmets? I ask because i am of the view that adults don't need a helmet, (backed up by research: http://www.cyclehelmets.org/index.html) and don't wear one myself, however i am aware that children have softer heads and might need padding. This is probably something I need to research further, but in the meantime does anyone have any experience or suggestions in this area?

thanks for any suggestions