We need a harness for our zip line for our 4 year olds that are 42 inches tall and weigh 53 lbs. I like the full boy harness, but am not sure which harness is best for them. Would you recommend the Simba or the O.? Thanks.

The simba would proably work best for a kid that size, especially considering they're going to keep growing. If you have smaller kids coming up you might consider the Petzl Ouistiti instead.

Ouitsi is better for the four year old. About age 5 or 6 the outgrow it and need the Simba.

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.