Even though Celebrity says the evening chic night only applies to the MDR it has been our experience that a majority of cruisers adhere to it in the specialty restaurants, Blu and Luminae. We also see a lot of guests staying dressed in the evening, especially with later dining. Personally, we just can't be bothered to change after dinner.
Unless you are cruising during certain times; spring break, holiday cruises, summer, there will be a lot fewer kids on Celebrity than Carnival. Longer cruises also have fewer kids.