In reply to Presley Whippet:
> At what age/size do kids move out of full body harnesses and into sit harnesses?
It's less by age and more by when they have developed enough hips that they can't fall out of a properly fitted sit harness if they go upside down. Typically it will be safe for a girl at a much younger age than a boy, but it very much depends on the individual.
If you're not sure about it, it might be a good idea to buy a sit harness and a separate chest harness, DMM do one, I'm sure others do. You tie in by going down through one chest harness loop, through the tie-in points on the sit harness, back up through the other chest harness loop and put the Fig 8 above that.