How big of a factor is stress in causing older children to wet the bed once in a while?

Depends. What form of stress are you discussing? I define "stress" as everythng that happens to us 24/7. E.G. Sweat when hot, shiver when cold etc. This is governed by the automatic control mechanisms in the brain. Enuresis is evidence of slow maturationj in this system and usually exposes dietary failure, but not necessarily.