I think you should add the other round. Not so much for size as the increase seems negligible to me but because that black bubble seems like the perfect finish of sequence for dark/light/dark/light/dark. Also as a child's comfort quilt I would be inclined to have a dark final round to better camouflage stains, dirt, etc from the dragging around part that always happens with well loved kids quilts. So YES the black bubble looks a perfect match to the rest. As far as backing, I'm a use whatever is available kind of gal for backings. I try to compliment but have been known to go totally scrappy or reversible on the backs. Go with what ever you like, either option sounds good.