The plain steel delta that ships with an NT harness is very good, the default leg pads are fine as is. I facilitate a lot of climbs and that's what everyone climbs on young and old. Stainless, pear screwlink or extended leg pads for example make sense if you have a specific need for them: stainless steel if you climb in the tropics, pear if you climb SRT and have extra gear to attach to your harness tie-in point, or extended leg pads if you anticipate long hang times for doing rigging or other in-tree activities that require you to be legs/feet off the tree and on the harness for extended periods.
If your son has some growing to do there is more reason to go basic since you'll buying another harness in the near future.
-moss