You can step up the reward, perhaps to a game of SuperSpeed, or Mind Soccer. They earn a say two minutes of play time a day. If they win every single day they have 10 minutes to play at the end of the week.
You could also reward them with 3 minutes of Mind Soccer play at the end of each day if they are winning, depending on the class. You would need to track where they were the day before, which should not be too hard.
That way you are rewarding small, but consistently and quickly, and they will likely work hard for it.