I am tutoring 3 kids in the same household of different math levels. 5th grade, pre-algebra, and algebra 1- ages 10, 12 and 14. The most common errors I notice in many kids is simple addition and subtraction errors.
A way that I have found most fun for the kids to practice their addition and subtraction, as well as probablility is by playing open-hand blackjack. This way, I as the dealer, can ask each of them questions like "How much do you need until you hit 21?" and "What probability do you have to get the card you need?" based on their math level.
They love the game, and we play it during their break time from their worksheets. I can't believe these kids who did not want to look at math more than they have to are begging to play blackjack during their break time!
So far, I think it has really improved all of their arithmetic!
Let's hope this continues! =)