Hafrael said:
I think YGO would also be good game to power level math skills. Very easy to get into, but can be a bit more competitive environment than MTG.
Not necessarily. Younger kids can have a difficult time grasping large numbers. Pokemon and Magic are definitely a lot easier to work with from a child's mind. Speaking from experience. When I'm not working at Gamestop on saturdays, I'll usually be teaching kids how to play the pokemon card game at this local game shop. Two years ago, I played with a three and a half year old girl who had just learned how to flip a coin. She didn't even know how to read. Her dad was over her shoulder, helping her along. She was having the time of her life, and was learning how to add, subtract, and multiply in the process. Even so, adding tens can get difficult for younger children.
I'm also hesitant to include YGO because some of the cards are inherently confusing. That's not to say magic or pokemon aren't, it's just that it's easier to keep the confusing cards to minimal levels in the other two mentioned games.