We had both a child seat and a trailer. We used them for different purposes.
I'd be loathe to use a child seat for anyone under 18months. Even with that, if you use a hlemet ensure that the back of the child seat accommodates it rather than pushing the head forwards (I am generally not in favour of helmets for under 2yo anyway as am unconvinced that they do not adversely affect the neck).
Here is a pic of our two oldest in a trailer. The baby seat is a cheap ABS car seat that had been trimmed to fit with a jigsaw and then padded with a karrimat. It is fitted with it's own restraining harness and is connected via a padding layer to the trailer (big bolts and washers, and multiple layers of karrimat)
img116 by
davidmamartin, on Flickr
The trailer is a Winther Dolphin which still remains as my most expensive single bike or bike-related purchase (more than I have ever spent on a new bike). Worth every penny though as it was used as a push chair, trailer and so on for about 8 years before being sold on.
