I wanted to circle back and give a quick follow up on the issue, and the MTDE Club harness for children, if for nothering else, posterity's sake perhaps.
The harness worked perfectly! It would be "even nicer" if it had more substantial gear loops, or riser straps, but those are just minor quibbles. It cinched down perfectly for my very slim 11-year-old daughter's waist & legs. The excess straps were kept at bay nicely, and it adjusted down snugly without issue.
If there's anyone needing a rugged harness for small-statured adults or children, I would give the MTDE Club harness a strong look.