1. Tiny study. 20 children is hardly a large sample.
2. The fact that the children enrolled in a Mahjong class sets them apart from most other children anyway. As such having a control group from the general population, rather than from people who might have enrolled for Mahjong but for some reason didn't, introduces a potential for bias.

Note, incidentally, that I'm not trying to discourage kids from playing Mahjong at that age. I did and look where it got me (!)