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P&O have two ships, Arcadia and Artemis, which are both child free. You could also try Saga. Or some of the more luxury lines like Seabourn, Silversea, Regent where although children are allowed they are not particularily catered for.
My experience on all P&P ships even with children on board, they are so well looked after in the Kids Club you hardly see them around the ship. It may be a good idea to avoid school holidays though.