In reply to Lynsety:
Sadly none going spare, however I do have to recommend the petzl full body one that comes with the added bonus of being brightly coloured and very easy to figure out how to put on.
I'd stick to full body harnesses with a chest attachment point, as apposed to a waist one, because of the potential for inversion being greater with a less experienced child, plus their hips won't yet have fully formed.
I have a 4 year old and a 2 year old that both get on with it fine