In reply to Billy Jeffs:
quick bit of googling, obviously not UIAA spec but something along these lines would presumably do the job for a bit of walking?
http://www.tesco.com/direct/littlelife-child-safety-harness/726-7281.prd?pa...
I remember being on reins/harness when I was a kid until much later than 2 (otherwise I wouldn't remember it!) although that was for use around roads rather than cliffs.
PPS further googling suggests better things are available (for a price) but I should probably do some work!
Post edited at 16:03