In reply to An Exiled Northerner:
Tie into both loops.
You want to keep the loop as short as possible, use the smallest knot you can and tie the stopper knot as close to the knot as possible - basically to minimise the chance of them getting hit in the face with a big lump of knot if they fall off.
I used to use a re-threaded fig-8 with the tail passed back through the knot, instead of an extra stopper knot.
I can't find a picture of it, but you end up with 3 strands coming out of the loop side and only 1 going away from the harness.
I preferred that to a bowline and stopper knot - I've done a lot of instructing with groups of kids.
Edit - see yellow rope, pics 3 and 4:
http://blog.alpineinstitute.com/2008/11/figure-eight-follow-through.htmlPost edited at 14:17