My 6.5 year old daughter just started roped climbing last week. She uses the Petzl full body harness. She may be much older than your daughter but she still only weighs 32 pounds so the harness will probably fit her in a similar fashion.
Good luck with it. They are never too young to get started climbing if they have the desire to do it.
(This post was edited by JasonsDrivingForce on Sep 24, 2012, 10:41 AM)