I took my girls to Great Keppel Island one year. It was great fun. Apart from the actual resort being fairly run down, the beaches are absolutely awesome (in terms of looks, but no surf or anything like that obviously) but a bit of a hike to get to sometimes. Also the jetski tour was sensational. Not many babes around, unless you're in to grannies. We met an older couple with kids while we were there, well older than us being 22 and 23 at the time, and they spent a few days on Hamilton Island before they went to Keppell and they said it's awesome for kids, so try that one