I am fairly confident GlasgowBlue's suggestion that the children's area should be empty will be correct and it is fairly soundproof where normal conversation is concermed. I would however suggest against this idea though as there is always that chance noisy children wanting to use the area for the correct usage will turn up and after all, this is the one designated area they are perfectly entitled to be children. It's difficult to say on any given day, but I would either go for the back left corner of the terrace in Galleries North, or as already stated the area overlooking security by the coffee machine in Glaaeries South.