Yep def a cover, and i always use an extra bamboo booster. I only use mine for night (only have 1) and use a fleece cover with it - no probs.
Darcy is 2 and a bit and I would guess at 15kg+. It is a bit snug, but she's never complained.
Don't know that it would be the only nappy I had in my stash though - it's so ugly!