Speak with your cruise agent, checkout the ship's websites. most of the bigger ships, like Millennium, do adults-only cruises. In our experience, sailing only with the bigger ships, children have never been a bother at all, completely marshalled by the various clubs & their minders in a very good fashion.
I imagine most cruise lines have an adult only ship, certainly Thomson and P & O do. We've sailed very successfully with both companies and no children, definitely my idea of fu.
We have tried p&o Arcadia and Artemis - both child-free ships. only adults allowed on minimum age 18. we have just returned from a wonderful time on Artemis which i can highly recommend out of these 2 P&O ships.
The P & O Arcadia is a superb shipand and is child free.
The only problem with child free ships is that if you are under 60 you will be in the minority.