We just returned from an extended-family (11 of us - 3 brothers in early 40's with 4 kids, 7 adults) Christmas cruise on Royal Caribbean's Serenade of the Seas.
We had 2 in their 60's, 4 in early 40's, 4 kids aged 8-14.
(Side note: suggestion: bring walkie talkies ($50 amazon for 3) if you have a group esp with kids and want to stay connected so you can find each other for meals or other times on board)
good marks here all around - we had four beds - 2 bunks that came down and 2 on the floor - they were all separated and I don't think we could push the 2 lower beds together though we didn't try. Couch, flat TV, closets bath and storage areas -- all very serviceable and proportional. We had a very large porthole/oceanview. Felt like the room was a smidge larger than similar rooms we've had on other ships though we book all kinds of rooms from interior to balcony - we did like this one.
Cozumel: Chankanaab park - $25 one way cab for 6, $21 admission. Then we sprang for the dolphin/manatee encounter which my wife expertly negotiated for $70 pp. The photos are where they really nail you - $194 for the digital DVD of all pics, also negotiated down by wife to $127 (all US$). Excursion from boat was like $120pp I think. Verdict: expensive either way if you do Chankanaab dolphins - not much difference there.