We don't have an issue with it not fitting either and DS1 is 9. We have a thick innerspring mattress on his bed too and it still covers pretty well - as i said doesn't look out of place at all. We use a single doona and just buy different doona covers. We just ordered the DS1 & DS2 a king single bunk beds and i got 2 x king single sheet sets and 2 x single quilt/doona cover sets for just $120 including postage from BIGW online.
Thanks hun!