Black Diamond Wiz Kid Harness - 2007 version. A blast. Girls and boys like it.
I have a 150" zipline in the backyard. My 6-8 year old girls use one
http://www.backcountry.com/store/PTZ0139/Petzl-Ouistiti-Full-Body-Climbing-Harness-Kids.html and http://www.backcountry.com/store/BLD0427/Black-Diamond-Wiz-Kid-Harness.html
I have had a big 3 year old comfortably fit into the Ouistiti. The girls flip upside down wearing either one and are very safe. Look also at the http://www.backcountry.com/store/MET0057/Metolius-PAS-Personal-Anchor-System-w-Element-Locking-Carabiner.html
as a easy means to adjust the distance from the ground or cable.
When designing my 145' zipline down my backyard hill for my kids and adults friends, I wanted safety a priority. When I found this PAS on Backcountry, I realized it perfect to customize the hook up distance for adults and children from the trolley. A bit more expensive than a single runner, but than 'one size fits all'. The added locking beiner is a no brainer.
My 5 yr old daughter loves this harness, feels safe as she spreads eagle zipping down our 145foot zipline to hit the bungeed stop block at the bottom. My 7 yr old dtr uses it as well, but has claimed the BD Whiz harness more comfortable for her; that or she sees it is more like Dad's Momentum.