I have 3 kids 6,4,&2yrs old. My 6yr old boy and 4 year old girl love to play in the trees. Really I have them on rope more than they are in the tree. I have been looking for a good kids harness, and I think you are right that the New Tribe would be a good choice. Also have been watching EBAY. Right now they both use harnesses that I made for them out of webbing -A "Swiss Seat" I think it is called 3 bowlines, one around each leg and one around the waist, grab all 3 loops with a biner and you are in there! I know that this is not ideal , but their "climbing" consists of me connecting them to the rope and hauling them up about 10-15 feet until they say stop, and then slowly lowering them down. They say the harness is comfy. I belay them with an 8.
I also frequently set up zip lines across the yard that they can play with on their own. I always make sure to have the low end bottom out at the ground rather than into the tree. This brings kids out from the whole street to play. Even sent a few parents flying.
Greg