At the setting with the cleanest and strongest turn over. I know it's not the answer your probably looking for. Another coach (men's coach) in my gym goes with the kids shoulder height. I don't like hard rules because I think it won't fit all kids.
We start on 115 or 120 (depending on the size of the child) and then click it up a notch when the entry looks correct. (no one competes on 115, I am talking about the beginning, most of my kids are on 125 and a few on 120. ) If they are coming in high I try to fix the entry.