Hi Fairy
Not exactly what you were asking for but I saw that you hadn't had an answer...
My sister's family went on a med cruise this Aug taking my mum and various other relatives with them. She has 2 DS - 2.5 and 5.5 (my sister, not my mum!)
By all accounts, they found it really relaxing and really enjoyed it but I seem to remember someone saying that there wasn't much for young children to do. Most of the ativities were geared up for older children and teens, eg table tennis, climbing walls, etc They also had to literally stop my younger nephew from throwing himself overboard a couple of times but he is VERY active and will never sit still. The 5 year old loves his food so he really enjoyed that aspect and he also spent a lot of time in the pool.
There were 6 adults and only the 2 DCs so there was always someone looking out for/entertaining them.
Also (under advice from someone else) they took a travel cot for the 2.5 yo. Whether this was just to stop him wandering off in the middle of the night, I'm not sure, but just check what kind of cots are on offer for your DCs.
Desperately trying to remember the cruise line that they went with - Princess something or other seems to ring a bell, though that might have been the name of the ship...
HTH