13oz probably wouldn't be too heavy but I would personally use a cheaper fabric for a childs kilt. (e.g. PV. ) PV is high quality but much lower cost and behaves very much like wool.

If you have the wool on hand though and don't mind using it for something that will be grown out of, give it a go.

About arisaids I think a lighter weight is used to decrease fatigue in the person wearing it. I have worn a heavy wool medieval cape for a day and it is alot of weight to lug around. (I happened to like it because it kept the rain off better than the poor souls wearing plastic ponchos.)