You need to tell us a bit more about age - that will really guide works for you and the child. Also, would you like the child sitting in the front or the back?
We have some co-pilot seats, and you could choose either of their versions.
Also, on my bike, we have a seat on the front which is now used by both children (2yr9m & 4yr6m):
http://www.ebay.co.uk/itm/Leco-Oxford-F ... 0567458430
The latter is dirt cheap, and I can't tell you how much the children like being in front, being able to really talk to you as you ride along.
BikeBiz has a good overview:
http://www.bikehub.co.uk/featured-artic ... -on-bikes/
And finally, if money were really no object, I'd get one of these:
http://www.urbanarrow.com/en Comes with electric assist if you want, is actually very, very easy to use, and is safe as houses. And you'd be the talk of the town in it!