Maths turns fun when it is learned through play. It is one of the convictions on which we build the educational activities in our after-school programme.
The reason is not that exercises are unpleasant in themselves, but that in maths the fear of being wrong appears early and lasts a long time. A child corrected a few times starts avoiding, and from there it is a short road to "I am no good at maths". Inside a game, a mistake does not carry the same weight: you lose a round and try again. Just as many exercises get solved, without the emotional cost that usually comes with them.
The second gain is rhythm. A game demands quick answers, and mental arithmetic is trained exactly that way: short, often, repeated. Long worksheets are not needed for a skill to form.
And what is left at the end is hard to measure but easy to notice: the children ask to play another round.