You might want to contact Bonnie Heather Greene who crafted a Dutch Friendship Tartan kilt for my then 3 year old son (aka "The Boy") that was designed to be somewhat expandable for future growth, with a hem and some extra fabric left folded into a couple of the pleats somewhere. A couple years later we sent it back to have her "let out" the extra fabric she had left strategically in and he got at least another year or so of use out of it before one more growth spirt left him either wearing a rather attractive miniskirt or pulling a serious "sag and bag" underwear showing low waist band look. So about a year ago (when he was 7) we had it retired along with his original infant kilt.

I can only imagine that a 12 year olds kilt would be larger to begin with and, depending on your family history of overall height and late growth or the lack thereof, would be easier to perform the same kind of magic with for a longer more durable and endurable design and build without sacrificing much. But Bonnie may have a few ideas. After ours being her first attempt at making a "growable" kilt, I remember her distinctly saying that the process of building ours led her to several new ideas on how to do it better in future similar kilts.