I took my 5 year old, and my friends 18mth on a Fred. Olsen Cruise last summer and I was apprehensive at first (thinking they were going to go overboard!). However it felt safer than any other type of holiday I've taken her on. The crew on Fred Olsen were amazing, letting them join in with them, and they had childrens clubs that they could both join in with.
So I would thoroughly recommend it, especially with children.