As far as I understand, civilians will not wear armor, and, furthermore, armor is the thing that's except from wear wear (but not combat wear), while all clothing is subjected to it. In addition to that, organic items are subjected to wear over time, so after 20 or so years your initial bags have been worn enough to reach "x" level.
Many types of clothing and armor are "common" in the mountain entity raws, which means that dwarven civs usually are able to make them, but they sometimes can be absent (if you're extremely unlucky you can make neither high nor low boots).
As skirts, kilts, etc. are items of clothing, I would have expected civilian dorfs to wear them when available, as well as exotic items such as face veils. However, I haven't performed any investigation into it.
Goblin and elven gear are compatible with dorfs, while human ones are too large for dorfs (but, obviously, suitable for your humies if you go for a multi racial fortress). Since dorfs are unable to make underwear, those articles of clothing have to be recovered from invaders or bought from elven caravans (and as I've engaged in trying to satisfy the dwarven demand for these items as well as I can, I'm completely convinced dorf wear items from both sources).
A big cause for kids playing on top of the corpses of slain goblins is that they went there to pick up "new" blood soaked underwear (since there's a constant shortage). Once that task is done, they're prone to decide it's time to play, and they decide to play where they happen to be at the time of the decision, go collect a toy, and return to the battlefield to play with the toy (and drop it there when done playing, for the next kid to pick it up and get a dose of corpse trauma).