Jen - if your nappies need covers, then go for wool. The wool absorbs the urine and allows it to evaporate, and they don't need washing every time (sounds gross, but put them in the sun if damp and all good) due to the lanolin in them. PUL covers are good for wearing clothes over, and fleece is sorta like wool but does trap the smell.

Yay Jen re the playgroup!