Contrary to other peoples opinions, I'd say just ask, there is no loss in asking. I have had a flight actually on CX (slightly more open in saying no) where it seemed one passenger was in business class, and his wife/kid was in economy class, but his kid came up to business class and sat with him for the entire journey down (on his lap, it was the 1-1-1 old flat bed seats). I found it nice of the crew to allow the child to do that, and wouldn't mind if that is in first even